drbd: bitmap keep track of changes vs on-disk bitmap

When we set or clear bits in a bitmap page,
also set a flag in the page->private pointer.

This allows us to skip writes of unchanged pages.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

6 files changed, 373 insertions, 220 deletions

diff --git a/drivers/block/drbd/drbd_actlog.c b/drivers/block/drbd/drbd_actlog.c
index e3f0f4d31d75..090fc2ce0df4 100644
--- a/drivers/block/drbd/drbd_actlog.c
+++ b/drivers/block/drbd/drbd_actlog.c
@@ -262,6 +262,33 @@ void drbd_al_complete_io(struct drbd_conf *mdev, sector_t sector)
         spin_unlock_irqrestore(&mdev->al_lock, flags);
 }
 
+#if (PAGE_SHIFT + 3) < (AL_EXTENT_SHIFT - BM_BLOCK_SHIFT)
+/* Currently BM_BLOCK_SHIFT, BM_EXT_SHIFT and AL_EXTENT_SHIFT
+ * are still coupled, or assume too much about their relation.
+ * Code below will not work if this is violated.
+ * Will be cleaned up with some followup patch.
+ */
+# error FIXME
+#endif
+
+static unsigned int al_extent_to_bm_page(unsigned int al_enr)
+{
+        return al_enr >>
+                /* bit to page */
+                ((PAGE_SHIFT + 3) -
+                /* al extent number to bit */
+                 (AL_EXTENT_SHIFT - BM_BLOCK_SHIFT));
+}
+
+static unsigned int rs_extent_to_bm_page(unsigned int rs_enr)
+{
+        return rs_enr >>
+                /* bit to page */
+                ((PAGE_SHIFT + 3) -
+                /* al extent number to bit */
+                 (BM_EXT_SHIFT - BM_BLOCK_SHIFT));
+}
+
 int
 w_al_write_transaction(struct drbd_conf *mdev, struct drbd_work *w, int unused)
 {
@@ -289,7 +316,7 @@ w_al_write_transaction(struct drbd_conf *mdev, struct drbd_work *w, int unused)
          * For now, we must not write the transaction,
          * if we cannot write out the bitmap of the evicted extent. */
         if (mdev->state.conn < C_CONNECTED && evicted != LC_FREE)
-                drbd_bm_write_sect(mdev, evicted/AL_EXT_PER_BM_SECT);
+                drbd_bm_write_page(mdev, al_extent_to_bm_page(evicted));
 
         /* The bitmap write may have failed, causing a state change. */
         if (mdev->state.disk < D_INCONSISTENT) {
@@ -636,105 +663,6 @@ out_bio_put:
 }
 
 /**
- * drbd_al_to_on_disk_bm() -  * Writes bitmap parts covered by active AL extents
- * @mdev:       DRBD device.
- *
- * Called when we detach (unconfigure) local storage,
- * or when we go from R_PRIMARY to R_SECONDARY role.
- */
-void drbd_al_to_on_disk_bm(struct drbd_conf *mdev)
-{
-        int i, nr_elements;
-        unsigned int enr;
-        struct bio **bios;
-        struct drbd_atodb_wait wc;
-
-        ERR_IF (!get_ldev_if_state(mdev, D_ATTACHING))
-                return; /* sorry, I don't have any act_log etc... */
-
-        wait_event(mdev->al_wait, lc_try_lock(mdev->act_log));
-
-        nr_elements = mdev->act_log->nr_elements;
-
-        /* GFP_KERNEL, we are not in anyone's write-out path */
-        bios = kzalloc(sizeof(struct bio *) * nr_elements, GFP_KERNEL);
-        if (!bios)
-                goto submit_one_by_one;
-
-        atomic_set(&wc.count, 0);
-        init_completion(&wc.io_done);
-        wc.mdev = mdev;
-        wc.error = 0;
-
-        for (i = 0; i < nr_elements; i++) {
-                enr = lc_element_by_index(mdev->act_log, i)->lc_number;
-                if (enr == LC_FREE)
-                        continue;
-                /* next statement also does atomic_inc wc.count and local_cnt */
-                if (atodb_prepare_unless_covered(mdev, bios,
-                                                enr/AL_EXT_PER_BM_SECT,
-                                                &wc))
-                        goto free_bios_submit_one_by_one;
-        }
-
-        /* unnecessary optimization? */
-        lc_unlock(mdev->act_log);
-        wake_up(&mdev->al_wait);
-
-        /* all prepared, submit them */
-        for (i = 0; i < nr_elements; i++) {
-                if (bios[i] == NULL)
-                        break;
-                if (drbd_insert_fault(mdev, DRBD_FAULT_MD_WR)) {
-                        bios[i]->bi_rw = WRITE;
-                        bio_endio(bios[i], -EIO);
-                } else {
-                        submit_bio(WRITE, bios[i]);
-                }
-        }
-
-        /* always (try to) flush bitmap to stable storage */
-        drbd_md_flush(mdev);
-
-        /* In case we did not submit a single IO do not wait for
-         * them to complete. ( Because we would wait forever here. )
-         *
-         * In case we had IOs and they are already complete, there
-         * is not point in waiting anyways.
-         * Therefore this if () ... */
-        if (atomic_read(&wc.count))
-                wait_for_completion(&wc.io_done);
-
-        put_ldev(mdev);
-
-        kfree(bios);
-        return;
-
- free_bios_submit_one_by_one:
-        /* free everything by calling the endio callback directly. */
-        for (i = 0; i < nr_elements && bios[i]; i++)
-                bio_endio(bios[i], 0);
-
-        kfree(bios);
-
- submit_one_by_one:
-        dev_warn(DEV, "Using the slow drbd_al_to_on_disk_bm()\n");
-
-        for (i = 0; i < mdev->act_log->nr_elements; i++) {
-                enr = lc_element_by_index(mdev->act_log, i)->lc_number;
-                if (enr == LC_FREE)
-                        continue;
-                /* Really slow: if we have al-extents 16..19 active,
-                 * sector 4 will be written four times! Synchronous! */
-                drbd_bm_write_sect(mdev, enr/AL_EXT_PER_BM_SECT);
-        }
-
-        lc_unlock(mdev->act_log);
-        wake_up(&mdev->al_wait);
-        put_ldev(mdev);
-}
-
-/**
  * drbd_al_apply_to_bm() - Sets the bitmap to diry(1) where covered ba active AL extents
  * @mdev:       DRBD device.
  */
@@ -813,7 +741,7 @@ static int w_update_odbm(struct drbd_conf *mdev, struct drbd_work *w, int unused
                 return 1;
         }
 
-        drbd_bm_write_sect(mdev, udw->enr);
+        drbd_bm_write_page(mdev, rs_extent_to_bm_page(udw->enr));
         put_ldev(mdev);
 
         kfree(udw);
@@ -893,7 +821,6 @@ static void drbd_try_clear_on_disk_bm(struct drbd_conf *mdev, sector_t sector,
                                 dev_warn(DEV, "Kicking resync_lru element enr=%u "
                                      "out with rs_failed=%d\n",
                                      ext->lce.lc_number, ext->rs_failed);
-                                set_bit(WRITE_BM_AFTER_RESYNC, &mdev->flags);
                         }
                         ext->rs_left = rs_left;
                         ext->rs_failed = success ? 0 : count;
@@ -912,7 +839,6 @@ static void drbd_try_clear_on_disk_bm(struct drbd_conf *mdev, sector_t sector,
                                 drbd_queue_work_front(&mdev->data.work, &udw->w);
                         } else {
                                 dev_warn(DEV, "Could not kmalloc an udw\n");
-                                set_bit(WRITE_BM_AFTER_RESYNC, &mdev->flags);
                         }
                 }
         } else {
diff --git a/drivers/block/drbd/drbd_bitmap.c b/drivers/block/drbd/drbd_bitmap.c
index 8d959ed6c2cc..72cd41a96ef9 100644
--- a/drivers/block/drbd/drbd_bitmap.c
+++ b/drivers/block/drbd/drbd_bitmap.c
@@ -70,8 +70,7 @@ struct drbd_bitmap {
         sector_t bm_dev_capacity;
         struct mutex bm_change; /* serializes resize operations */
 
-        atomic_t bm_async_io;
-        wait_queue_head_t bm_io_wait;
+        wait_queue_head_t bm_io_wait; /* used to serialize IO of single pages */
 
         unsigned long  bm_flags;
 
@@ -82,7 +81,7 @@ struct drbd_bitmap {
 
 /* definition of bits in bm_flags */
 #define BM_LOCKED       0
-#define BM_MD_IO_ERROR  1
+// #define BM_MD_IO_ERROR  1 unused now.
 #define BM_P_VMALLOCED  2
 
 static int __bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
@@ -155,26 +154,117 @@ void drbd_bm_unlock(struct drbd_conf *mdev)
         mutex_unlock(&b->bm_change);
 }
 
-static unsigned int bm_bit_to_page_idx(struct drbd_bitmap *b, u64 bitnr)
+/* we store some "meta" info about our pages in page->private */
+/* at a granularity of 4k storage per bitmap bit:
+ * one peta byte storage: 1<<50 byte, 1<<38 * 4k storage blocks
+ *  1<<38 bits,
+ *  1<<23 4k bitmap pages.
+ * Use 24 bits as page index, covers 2 peta byte storage
+ * at a granularity of 4k per bit.
+ * Used to report the failed page idx on io error from the endio handlers.
+ */
+#define BM_PAGE_IDX_MASK        ((1UL<<24)-1)
+/* this page is currently read in, or written back */
+#define BM_PAGE_IO_LOCK         31
+/* if there has been an IO error for this page */
+#define BM_PAGE_IO_ERROR        30
+/* this is to be able to intelligently skip disk IO,
+ * set if bits have been set since last IO. */
+#define BM_PAGE_NEED_WRITEOUT   29
+/* to mark for lazy writeout once syncer cleared all clearable bits,
+ * we if bits have been cleared since last IO. */
+#define BM_PAGE_LAZY_WRITEOUT   28
+
+/* store_page_idx uses non-atomic assingment. It is only used directly after
+ * allocating the page.  All other bm_set_page_* and bm_clear_page_* need to
+ * use atomic bit manipulation, as set_out_of_sync (and therefore bitmap
+ * changes) may happen from various contexts, and wait_on_bit/wake_up_bit
+ * requires it all to be atomic as well. */
+static void bm_store_page_idx(struct page *page, unsigned long idx)
 {
-       /* page_nr = (bitnr/8) >> PAGE_SHIFT; */
-       unsigned int page_nr = bitnr >> (PAGE_SHIFT + 3);
-       BUG_ON(page_nr >= b->bm_number_of_pages);
-       return page_nr;
+        BUG_ON(0 != (idx & ~BM_PAGE_IDX_MASK));
+        page_private(page) |= idx;
 }
 
-/* word offset to long pointer */
-static unsigned long *__bm_map_paddr(struct drbd_bitmap *b, unsigned long offset, const enum km_type km)
+static unsigned long bm_page_to_idx(struct page *page)
 {
-        struct page *page;
-        unsigned long page_nr;
+        return page_private(page) & BM_PAGE_IDX_MASK;
+}
+
+/* As is very unlikely that the same page is under IO from more than one
+ * context, we can get away with a bit per page and one wait queue per bitmap.
+ */
+static void bm_page_lock_io(struct drbd_conf *mdev, int page_nr)
+{
+        struct drbd_bitmap *b = mdev->bitmap;
+        void *addr = &page_private(b->bm_pages[page_nr]);
+        wait_event(b->bm_io_wait, !test_and_set_bit(BM_PAGE_IO_LOCK, addr));
+}
+
+static void bm_page_unlock_io(struct drbd_conf *mdev, int page_nr)
+{
+        struct drbd_bitmap *b = mdev->bitmap;
+        void *addr = &page_private(b->bm_pages[page_nr]);
+        clear_bit(BM_PAGE_IO_LOCK, addr);
+        smp_mb__after_clear_bit();
+        wake_up(&mdev->bitmap->bm_io_wait);
+}
+
+/* set _before_ submit_io, so it may be reset due to being changed
+ * while this page is in flight... will get submitted later again */
+static void bm_set_page_unchanged(struct page *page)
+{
+        /* use cmpxchg? */
+        clear_bit(BM_PAGE_NEED_WRITEOUT, &page_private(page));
+        clear_bit(BM_PAGE_LAZY_WRITEOUT, &page_private(page));
+}
+
+static void bm_set_page_need_writeout(struct page *page)
+{
+        set_bit(BM_PAGE_NEED_WRITEOUT, &page_private(page));
+}
+
+static int bm_test_page_unchanged(struct page *page)
+{
+        volatile const unsigned long *addr = &page_private(page);
+        return (*addr & ((1UL<<BM_PAGE_NEED_WRITEOUT)|(1UL<<BM_PAGE_LAZY_WRITEOUT))) == 0;
+}
 
+static void bm_set_page_io_err(struct page *page)
+{
+        set_bit(BM_PAGE_IO_ERROR, &page_private(page));
+}
+
+static void bm_clear_page_io_err(struct page *page)
+{
+        clear_bit(BM_PAGE_IO_ERROR, &page_private(page));
+}
+
+static void bm_set_page_lazy_writeout(struct page *page)
+{
+        set_bit(BM_PAGE_LAZY_WRITEOUT, &page_private(page));
+}
+
+static int bm_test_page_lazy_writeout(struct page *page)
+{
+        return test_bit(BM_PAGE_LAZY_WRITEOUT, &page_private(page));
+}
+
+/* on a 32bit box, this would allow for exactly (2<<38) bits. */
+static unsigned int bm_word_to_page_idx(struct drbd_bitmap *b, unsigned long long_nr)
+{
         /* page_nr = (word*sizeof(long)) >> PAGE_SHIFT; */
-        page_nr = offset >> (PAGE_SHIFT - LN2_BPL + 3);
+        unsigned int page_nr = long_nr >> (PAGE_SHIFT - LN2_BPL + 3);
         BUG_ON(page_nr >= b->bm_number_of_pages);
-        page = b->bm_pages[page_nr];
+        return page_nr;
+}
 
-        return (unsigned long *) kmap_atomic(page, km);
+static unsigned int bm_bit_to_page_idx(struct drbd_bitmap *b, u64 bitnr)
+{
+        /* page_nr = (bitnr/8) >> PAGE_SHIFT; */
+        unsigned int page_nr = bitnr >> (PAGE_SHIFT + 3);
+        BUG_ON(page_nr >= b->bm_number_of_pages);
+        return page_nr;
 }
 
 static unsigned long *__bm_map_pidx(struct drbd_bitmap *b, unsigned int idx, const enum km_type km)
@@ -188,11 +278,6 @@ static unsigned long *bm_map_pidx(struct drbd_bitmap *b, unsigned int idx)
         return __bm_map_pidx(b, idx, KM_IRQ1);
 }
 
-static unsigned long * bm_map_paddr(struct drbd_bitmap *b, unsigned long offset)
-{
-        return __bm_map_paddr(b, offset, KM_IRQ1);
-}
-
 static void __bm_unmap(unsigned long *p_addr, const enum km_type km)
 {
         kunmap_atomic(p_addr, km);
@@ -222,6 +307,7 @@ static void bm_unmap(unsigned long *p_addr)
  * to be able to report device specific.
  */
 
+
 static void bm_free_pages(struct page **pages, unsigned long number)
 {
         unsigned long i;
@@ -289,6 +375,9 @@ static struct page **bm_realloc_pages(struct drbd_bitmap *b, unsigned long want)
                                 bm_vk_free(new_pages, vmalloced);
                                 return NULL;
                         }
+                        /* we want to know which page it is
+                         * from the endio handlers */
+                        bm_store_page_idx(page, i);
                         new_pages[i] = page;
                 }
         } else {
@@ -443,7 +532,7 @@ static unsigned long bm_count_bits(struct drbd_bitmap *b)
 
         while (offset < words) {
                 i = do_now = min_t(size_t, words-offset, LWPP);
-                p_addr = __bm_map_paddr(b, offset, KM_USER0);
+                p_addr = __bm_map_pidx(b, bm_word_to_page_idx(b, offset), KM_USER0);
                 bm = p_addr + MLPP(offset);
                 while (i--) {
                         bits += hweight_long(*bm++);
@@ -472,6 +561,7 @@ static unsigned long bm_count_bits(struct drbd_bitmap *b)
 static void bm_memset(struct drbd_bitmap *b, size_t offset, int c, size_t len)
 {
         unsigned long *p_addr, *bm;
+        unsigned int idx;
         size_t do_now, end;
 
 #define BM_SECTORS_PER_BIT (BM_BLOCK_SIZE/512)
@@ -485,7 +575,8 @@ static void bm_memset(struct drbd_bitmap *b, size_t offset, int c, size_t len)
 
         while (offset < end) {
                 do_now = min_t(size_t, ALIGN(offset + 1, LWPP), end) - offset;
-                p_addr = bm_map_paddr(b, offset);
+                idx = bm_word_to_page_idx(b, offset);
+                p_addr = bm_map_pidx(b, idx);
                 bm = p_addr + MLPP(offset);
                 if (bm+do_now > p_addr + LWPP) {
                         printk(KERN_ALERT "drbd: BUG BUG BUG! p_addr:%p bm:%p do_now:%d\n",
@@ -494,6 +585,7 @@ static void bm_memset(struct drbd_bitmap *b, size_t offset, int c, size_t len)
                 }
                 memset(bm, c, do_now * sizeof(long));
                 bm_unmap(p_addr);
+                bm_set_page_need_writeout(b->bm_pages[idx]);
                 offset += do_now;
         }
 }
@@ -604,7 +696,7 @@ int drbd_bm_resize(struct drbd_conf *mdev, sector_t capacity, int set_new_bits)
                 bm_free_pages(opages + want, have - want);
         }
 
-        p_addr = bm_map_paddr(b, words);
+        p_addr = bm_map_pidx(b, bm_word_to_page_idx(b, words));
         bm = p_addr + MLPP(words);
         *bm = DRBD_MAGIC;
         bm_unmap(p_addr);
@@ -616,7 +708,7 @@ int drbd_bm_resize(struct drbd_conf *mdev, sector_t capacity, int set_new_bits)
                 bm_vk_free(opages, opages_vmalloced);
         if (!growing)
                 b->bm_set = bm_count_bits(b);
-        dev_info(DEV, "resync bitmap: bits=%lu words=%lu\n", bits, words);
+        dev_info(DEV, "resync bitmap: bits=%lu words=%lu pages=%lu\n", bits, words, want);
 
  out:
         drbd_bm_unlock(mdev);
@@ -686,6 +778,7 @@ void drbd_bm_merge_lel(struct drbd_conf *mdev, size_t offset, size_t number,
         struct drbd_bitmap *b = mdev->bitmap;
         unsigned long *p_addr, *bm;
         unsigned long word, bits;
+        unsigned int idx;
         size_t end, do_now;
 
         end = offset + number;
@@ -700,7 +793,8 @@ void drbd_bm_merge_lel(struct drbd_conf *mdev, size_t offset, size_t number,
         spin_lock_irq(&b->bm_lock);
         while (offset < end) {
                 do_now = min_t(size_t, ALIGN(offset+1, LWPP), end) - offset;
-                p_addr = bm_map_paddr(b, offset);
+                idx = bm_word_to_page_idx(b, offset);
+                p_addr = bm_map_pidx(b, idx);
                 bm = p_addr + MLPP(offset);
                 offset += do_now;
                 while (do_now--) {
@@ -710,6 +804,7 @@ void drbd_bm_merge_lel(struct drbd_conf *mdev, size_t offset, size_t number,
                         b->bm_set += hweight_long(word) - bits;
                 }
                 bm_unmap(p_addr);
+                bm_set_page_need_writeout(b->bm_pages[idx]);
         }
         /* with 32bit <-> 64bit cross-platform connect
          * this is only correct for current usage,
@@ -748,7 +843,7 @@ void drbd_bm_get_lel(struct drbd_conf *mdev, size_t offset, size_t number,
         else {
                 while (offset < end) {
                         do_now = min_t(size_t, ALIGN(offset+1, LWPP), end) - offset;
-                        p_addr = bm_map_paddr(b, offset);
+                        p_addr = bm_map_pidx(b, bm_word_to_page_idx(b, offset));
                         bm = p_addr + MLPP(offset);
                         offset += do_now;
                         while (do_now--)
@@ -786,9 +881,22 @@ void drbd_bm_clear_all(struct drbd_conf *mdev)
         spin_unlock_irq(&b->bm_lock);
 }
 
+struct bm_aio_ctx {
+        struct drbd_conf *mdev;
+        atomic_t in_flight;
+        wait_queue_head_t io_wait;
+        unsigned flags;
+#define BM_AIO_COPY_PAGES       1
+        int error;
+};
+
+/* bv_page may be a copy, or may be the original */
 static void bm_async_io_complete(struct bio *bio, int error)
 {
-        struct drbd_bitmap *b = bio->bi_private;
+        struct bm_aio_ctx *ctx = bio->bi_private;
+        struct drbd_conf *mdev = ctx->mdev;
+        struct drbd_bitmap *b = mdev->bitmap;
+        unsigned int idx = bm_page_to_idx(bio->bi_io_vec[0].bv_page);
         int uptodate = bio_flagged(bio, BIO_UPTODATE);
 
 
@@ -799,35 +907,79 @@ static void bm_async_io_complete(struct bio *bio, int error)
         if (!error && !uptodate)
                 error = -EIO;
 
+        if (!bm_test_page_unchanged(b->bm_pages[idx]))
+                dev_info(DEV, "bitmap page idx %u changed during IO!\n", idx);
+
         if (error) {
-                /* doh. what now?
-                 * for now, set all bits, and flag MD_IO_ERROR */
-                __set_bit(BM_MD_IO_ERROR, &b->bm_flags);
+                /* ctx error will hold the completed-last non-zero error code,
+                 * in case error codes differ. */
+                ctx->error = error;
+                bm_set_page_io_err(b->bm_pages[idx]);
+                /* Not identical to on disk version of it.
+                 * Is BM_PAGE_IO_ERROR enough? */
+                if (__ratelimit(&drbd_ratelimit_state))
+                        dev_err(DEV, "IO ERROR %d on bitmap page idx %u\n",
+                                        error, idx);
+        } else {
+                bm_clear_page_io_err(b->bm_pages[idx]);
+                dynamic_dev_dbg(DEV, "bitmap page idx %u completed\n", idx);
         }
-        if (atomic_dec_and_test(&b->bm_async_io))
-                wake_up(&b->bm_io_wait);
+
+        bm_page_unlock_io(mdev, idx);
+
+        /* FIXME give back to page pool */
+        if (ctx->flags & BM_AIO_COPY_PAGES)
+                put_page(bio->bi_io_vec[0].bv_page);
 
         bio_put(bio);
+
+        if (atomic_dec_and_test(&ctx->in_flight))
+                wake_up(&ctx->io_wait);
 }
 
-static void bm_page_io_async(struct drbd_conf *mdev, struct drbd_bitmap *b, int page_nr, int rw) __must_hold(local)
+static void bm_page_io_async(struct bm_aio_ctx *ctx, int page_nr, int rw) __must_hold(local)
 {
         /* we are process context. we always get a bio */
         struct bio *bio = bio_alloc(GFP_KERNEL, 1);
+        struct drbd_conf *mdev = ctx->mdev;
+        struct drbd_bitmap *b = mdev->bitmap;
+        struct page *page;
         unsigned int len;
+
         sector_t on_disk_sector =
                 mdev->ldev->md.md_offset + mdev->ldev->md.bm_offset;
         on_disk_sector += ((sector_t)page_nr) << (PAGE_SHIFT-9);
 
         /* this might happen with very small
-         * flexible external meta data device */
+         * flexible external meta data device,
+         * or with PAGE_SIZE > 4k */
         len = min_t(unsigned int, PAGE_SIZE,
                 (drbd_md_last_sector(mdev->ldev) - on_disk_sector + 1)<<9);
 
+        /* serialize IO on this page */
+        bm_page_lock_io(mdev, page_nr);
+        /* before memcpy and submit,
+         * so it can be redirtied any time */
+        bm_set_page_unchanged(b->bm_pages[page_nr]);
+
+        if (ctx->flags & BM_AIO_COPY_PAGES) {
+                /* FIXME alloc_page is good enough for now, but actually needs
+                 * to use pre-allocated page pool */
+                void *src, *dest;
+                page = alloc_page(__GFP_HIGHMEM|__GFP_WAIT);
+                dest = kmap_atomic(page, KM_USER0);
+                src = kmap_atomic(b->bm_pages[page_nr], KM_USER1);
+                memcpy(dest, src, PAGE_SIZE);
+                kunmap_atomic(src, KM_USER1);
+                kunmap_atomic(dest, KM_USER0);
+                bm_store_page_idx(page, page_nr);
+        } else
+                page = b->bm_pages[page_nr];
+
         bio->bi_bdev = mdev->ldev->md_bdev;
         bio->bi_sector = on_disk_sector;
-        bio_add_page(bio, b->bm_pages[page_nr], len, 0);
-        bio->bi_private = b;
+        bio_add_page(bio, page, len, 0);
+        bio->bi_private = ctx;
         bio->bi_end_io = bm_async_io_complete;
 
         if (drbd_insert_fault(mdev, (rw & WRITE) ? DRBD_FAULT_MD_WR : DRBD_FAULT_MD_RD)) {
@@ -841,36 +993,72 @@ static void bm_page_io_async(struct drbd_conf *mdev, struct drbd_bitmap *b, int
 /*
  * bm_rw: read/write the whole bitmap from/to its on disk location.
  */
-static int bm_rw(struct drbd_conf *mdev, int rw) __must_hold(local)
+static int bm_rw(struct drbd_conf *mdev, int rw, unsigned lazy_writeout_upper_idx) __must_hold(local)
 {
+        struct bm_aio_ctx ctx =
+                { .flags = lazy_writeout_upper_idx ? BM_AIO_COPY_PAGES : 0 };
         struct drbd_bitmap *b = mdev->bitmap;
-        /* sector_t sector; */
-        int bm_words, num_pages, i;
+        int last_page, i, count = 0;
         unsigned long now;
         char ppb[10];
         int err = 0;
 
-        WARN_ON(!bm_is_locked(b));
-
-        /* no spinlock here, the drbd_bm_lock should be enough! */
+        /*
+         * We are protected against bitmap disappearing/resizing by holding an
+         * ldev reference (caller must have called get_ldev()).
+         * For read/write, we are protected against changes to the bitmap by
+         * the bitmap lock (see drbd_bitmap_io).
+         * For lazy writeout, we don't care for ongoing changes to the bitmap,
+         * as we submit copies of pages anyways.
+         */
+        if (!ctx.flags)
+                WARN_ON(!bm_is_locked(b));
 
-        bm_words  = drbd_bm_words(mdev);
-        num_pages = (bm_words*sizeof(long) + PAGE_SIZE-1) >> PAGE_SHIFT;
+        /* because of the "extra long to catch oob access" we allocate in
+         * drbd_bm_resize, bm_number_of_pages -1 is not necessarily the page
+         * containing the last _relevant_ bitmap word */
+        last_page = bm_word_to_page_idx(b, b->bm_words - 1);
 
         now = jiffies;
-        atomic_set(&b->bm_async_io, num_pages);
-        __clear_bit(BM_MD_IO_ERROR, &b->bm_flags);
+        ctx.mdev = mdev;
+        atomic_set(&ctx.in_flight, 1); /* one extra ref */
+        init_waitqueue_head(&ctx.io_wait);
+        ctx.error = 0;
 
         /* let the layers below us try to merge these bios... */
-        for (i = 0; i < num_pages; i++)
-                bm_page_io_async(mdev, b, i, rw);
+        for (i = 0; i <= last_page; i++) {
+                /* ignore completely unchanged pages */
+                if (lazy_writeout_upper_idx && i == lazy_writeout_upper_idx)
+                        break;
+                if (rw & WRITE) {
+                        if (bm_test_page_unchanged(b->bm_pages[i])) {
+                                dynamic_dev_dbg(DEV, "skipped bm write for idx %u\n", i);
+                                continue;
+                        }
+                        /* during lazy writeout,
+                         * ignore those pages not marked for lazy writeout. */
+                        if (lazy_writeout_upper_idx &&
+                            !bm_test_page_lazy_writeout(b->bm_pages[i])) {
+                                dynamic_dev_dbg(DEV, "skipped bm lazy write for idx %u\n", i);
+                                continue;
+                        }
+                }
+                atomic_inc(&ctx.in_flight);
+                bm_page_io_async(&ctx, i, rw);
+                ++count;
+                cond_resched();
+        }
 
-        wait_event(b->bm_io_wait, atomic_read(&b->bm_async_io) == 0);
+        atomic_dec(&ctx.in_flight); /* drop the extra ref */
+        wait_event(ctx.io_wait, atomic_read(&ctx.in_flight) == 0);
+        dev_info(DEV, "bitmap %s of %u pages took %lu jiffies\n",
+                        rw == WRITE ? "WRITE" : "READ",
+                        count, jiffies - now);
 
-        if (test_bit(BM_MD_IO_ERROR, &b->bm_flags)) {
+        if (ctx.error) {
                 dev_alert(DEV, "we had at least one MD IO ERROR during bitmap IO\n");
                 drbd_chk_io_error(mdev, 1, true);
-                err = -EIO;
+                err = -EIO; /* ctx.error ? */
         }
 
         now = jiffies;
@@ -895,55 +1083,63 @@ static int bm_rw(struct drbd_conf *mdev, int rw) __must_hold(local)
  */
 int drbd_bm_read(struct drbd_conf *mdev) __must_hold(local)
 {
-        return bm_rw(mdev, READ);
+        return bm_rw(mdev, READ, 0);
 }
 
 /**
  * drbd_bm_write() - Write the whole bitmap to its on disk location.
  * @mdev:       DRBD device.
+ *
+ * Will only write pages that have changed since last IO.
  */
 int drbd_bm_write(struct drbd_conf *mdev) __must_hold(local)
 {
-        return bm_rw(mdev, WRITE);
+        return bm_rw(mdev, WRITE, 0);
 }
 
 /**
- * drbd_bm_write_sect: Writes a 512 (MD_SECTOR_SIZE) byte piece of the bitmap
+ * drbd_bm_lazy_write_out() - Write bitmap pages 0 to @upper_idx-1, if they have changed.
  * @mdev:       DRBD device.
- * @enr:        Extent number in the resync lru (happens to be sector offset)
+ * @upper_idx:  0: write all changed pages; +ve: page index to stop scanning for changed pages
+ */
+int drbd_bm_write_lazy(struct drbd_conf *mdev, unsigned upper_idx) __must_hold(local)
+{
+        return bm_rw(mdev, WRITE, upper_idx);
+}
+
+
+/**
+ * drbd_bm_write_page: Writes a PAGE_SIZE aligned piece of bitmap
+ * @mdev:       DRBD device.
+ * @idx:        bitmap page index
  *
- * The BM_EXT_SIZE is on purpose exactly the amount of the bitmap covered
- * by a single sector write. Therefore enr == sector offset from the
- * start of the bitmap.
+ * We don't want to special case on logical_block_size of the underlaying
+ * device, so we submit PAGE_SIZE aligned pieces containing the requested enr.
+ * Note that on "most" systems, PAGE_SIZE is 4k.
  */
-int drbd_bm_write_sect(struct drbd_conf *mdev, unsigned long enr) __must_hold(local)
+int drbd_bm_write_page(struct drbd_conf *mdev, unsigned int idx) __must_hold(local)
 {
-        sector_t on_disk_sector = enr + mdev->ldev->md.md_offset
-                                      + mdev->ldev->md.bm_offset;
-        int bm_words, num_words, offset;
-        int err = 0;
+        struct bm_aio_ctx ctx = { .flags = BM_AIO_COPY_PAGES, };
 
-        mutex_lock(&mdev->md_io_mutex);
-        bm_words  = drbd_bm_words(mdev);
-        offset    = S2W(enr);   /* word offset into bitmap */
-        num_words = min(S2W(1), bm_words - offset);
-        if (num_words < S2W(1))
-                memset(page_address(mdev->md_io_page), 0, MD_SECTOR_SIZE);
-        drbd_bm_get_lel(mdev, offset, num_words,
-                        page_address(mdev->md_io_page));
-        if (!drbd_md_sync_page_io(mdev, mdev->ldev, on_disk_sector, WRITE)) {
-                int i;
-                err = -EIO;
-                dev_err(DEV, "IO ERROR writing bitmap sector %lu "
-                    "(meta-disk sector %llus)\n",
-                    enr, (unsigned long long)on_disk_sector);
-                drbd_chk_io_error(mdev, 1, true);
-                for (i = 0; i < AL_EXT_PER_BM_SECT; i++)
-                        drbd_bm_ALe_set_all(mdev, enr*AL_EXT_PER_BM_SECT+i);
+        if (bm_test_page_unchanged(mdev->bitmap->bm_pages[idx])) {
+                dev_info(DEV, "skipped bm page write for idx %u\n", idx);
+                return 0;
         }
+
+        ctx.mdev = mdev;
+        atomic_set(&ctx.in_flight, 1);
+        init_waitqueue_head(&ctx.io_wait);
+
+        bm_page_io_async(&ctx, idx, WRITE_SYNC);
+        wait_event(ctx.io_wait, atomic_read(&ctx.in_flight) == 0);
+
+        if (ctx.error)
+                drbd_chk_io_error(mdev, 1, true);
+                /* that should force detach, so the in memory bitmap will be
+                 * gone in a moment as well. */
+
         mdev->bm_writ_cnt++;
-        mutex_unlock(&mdev->md_io_mutex);
-        return err;
+        return ctx.error;
 }
 
 /* NOTE
@@ -965,10 +1161,9 @@ static unsigned long __bm_find_next(struct drbd_conf *mdev, unsigned long bm_fo,
                 dev_err(DEV, "bm_fo=%lu bm_bits=%lu\n", bm_fo, b->bm_bits);
         } else {
                 while (bm_fo < b->bm_bits) {
-                        unsigned long offset;
-                        bit_offset = bm_fo & ~BPP_MASK; /* bit offset of the page */
-                        offset = bit_offset >> LN2_BPL;    /* word offset of the page */
-                        p_addr = __bm_map_paddr(b, offset, km);
+                        /* bit offset of the first bit in the page */
+                        bit_offset = bm_fo & ~BPP_MASK;
+                        p_addr = __bm_map_pidx(b, bm_bit_to_page_idx(b, bm_fo), km);
 
                         if (find_zero_bit)
                                 i = generic_find_next_zero_le_bit(p_addr, PAGE_SIZE*8, bm_fo & BPP_MASK);
@@ -1048,8 +1243,9 @@ static int __bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
         struct drbd_bitmap *b = mdev->bitmap;
         unsigned long *p_addr = NULL;
         unsigned long bitnr;
-        unsigned long last_page_nr = -1UL;
+        unsigned int last_page_nr = -1U;
         int c = 0;
+        int changed_total = 0;
 
         if (e >= b->bm_bits) {
                 dev_err(DEV, "ASSERT FAILED: bit_s=%lu bit_e=%lu bm_bits=%lu\n",
@@ -1057,12 +1253,17 @@ static int __bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
                 e = b->bm_bits ? b->bm_bits -1 : 0;
         }
         for (bitnr = s; bitnr <= e; bitnr++) {
-                unsigned long offset = bitnr>>LN2_BPL;
-                unsigned long page_nr = offset >> (PAGE_SHIFT - LN2_BPL + 3);
+                unsigned int page_nr = bm_bit_to_page_idx(b, bitnr);
                 if (page_nr != last_page_nr) {
                         if (p_addr)
                                 __bm_unmap(p_addr, km);
-                        p_addr = __bm_map_paddr(b, offset, km);
+                        if (c < 0)
+                                bm_set_page_lazy_writeout(b->bm_pages[last_page_nr]);
+                        else if (c > 0)
+                                bm_set_page_need_writeout(b->bm_pages[last_page_nr]);
+                        changed_total += c;
+                        c = 0;
+                        p_addr = __bm_map_pidx(b, page_nr, km);
                         last_page_nr = page_nr;
                 }
                 if (val)
@@ -1072,8 +1273,13 @@ static int __bm_change_bits_to(struct drbd_conf *mdev, const unsigned long s,
         }
         if (p_addr)
                 __bm_unmap(p_addr, km);
-        b->bm_set += c;
-        return c;
+        if (c < 0)
+                bm_set_page_lazy_writeout(b->bm_pages[last_page_nr]);
+        else if (c > 0)
+                bm_set_page_need_writeout(b->bm_pages[last_page_nr]);
+        changed_total += c;
+        b->bm_set += changed_total;
+        return changed_total;
 }
 
 /* returns number of bits actually changed.
@@ -1211,8 +1417,7 @@ int drbd_bm_test_bit(struct drbd_conf *mdev, const unsigned long bitnr)
         if (bm_is_locked(b))
                 bm_print_lock_info(mdev);
         if (bitnr < b->bm_bits) {
-                unsigned long offset = bitnr>>LN2_BPL;
-                p_addr = bm_map_paddr(b, offset);
+                p_addr = bm_map_pidx(b, bm_bit_to_page_idx(b, bitnr));
                 i = generic_test_le_bit(bitnr & BPP_MASK, p_addr) ? 1 : 0;
                 bm_unmap(p_addr);
         } else if (bitnr == b->bm_bits) {
@@ -1231,10 +1436,10 @@ int drbd_bm_count_bits(struct drbd_conf *mdev, const unsigned long s, const unsi
 {
         unsigned long flags;
         struct drbd_bitmap *b = mdev->bitmap;
-        unsigned long *p_addr = NULL, page_nr = -1;
+        unsigned long *p_addr = NULL;
         unsigned long bitnr;
+        unsigned int page_nr = -1U;
         int c = 0;
-        size_t w;
 
         /* If this is called without a bitmap, that is a bug.  But just to be
          * robust in case we screwed up elsewhere, in that case pretend there
@@ -1247,12 +1452,12 @@ int drbd_bm_count_bits(struct drbd_conf *mdev, const unsigned long s, const unsi
         if (bm_is_locked(b))
                 bm_print_lock_info(mdev);
         for (bitnr = s; bitnr <= e; bitnr++) {
-                w = bitnr >> LN2_BPL;
-                if (page_nr != w >> (PAGE_SHIFT - LN2_BPL + 3)) {
-                        page_nr = w >> (PAGE_SHIFT - LN2_BPL + 3);
+                unsigned int idx = bm_bit_to_page_idx(b, bitnr);
+                if (page_nr != idx) {
+                        page_nr = idx;
                         if (p_addr)
                                 bm_unmap(p_addr);
-                        p_addr = bm_map_paddr(b, w);
+                        p_addr = bm_map_pidx(b, idx);
                 }
                 ERR_IF (bitnr >= b->bm_bits) {
                         dev_err(DEV, "bitnr=%lu bm_bits=%lu\n", bitnr, b->bm_bits);
@@ -1300,7 +1505,7 @@ int drbd_bm_e_weight(struct drbd_conf *mdev, unsigned long enr)
         count = 0;
         if (s < b->bm_words) {
                 int n = e-s;
-                p_addr = bm_map_paddr(b, s);
+                p_addr = bm_map_pidx(b, bm_word_to_page_idx(b, s));
                 bm = p_addr + MLPP(s);
                 while (n--)
                         count += hweight_long(*bm++);
@@ -1335,7 +1540,7 @@ unsigned long drbd_bm_ALe_set_all(struct drbd_conf *mdev, unsigned long al_enr)
         count = 0;
         if (s < b->bm_words) {
                 i = do_now = e-s;
-                p_addr = bm_map_paddr(b, s);
+                p_addr = bm_map_pidx(b, bm_word_to_page_idx(b, s));
                 bm = p_addr + MLPP(s);
                 while (i--) {
                         count += hweight_long(*bm);
diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
index f6da48bb8c70..74cc50a21822 100644
--- a/drivers/block/drbd/drbd_int.h
+++ b/drivers/block/drbd/drbd_int.h
@@ -833,7 +833,7 @@ enum {
         CRASHED_PRIMARY,        /* This node was a crashed primary.
                                  * Gets cleared when the state.conn
                                  * goes into C_CONNECTED state. */
-        WRITE_BM_AFTER_RESYNC,  /* A kmalloc() during resync failed */
+        NO_BARRIER_SUPP,        /* underlying block device doesn't implement barriers */
         CONSIDER_RESYNC,
 
         MD_NO_FUA,              /* Users wants us to not use FUA/FLUSH on meta data dev */
@@ -1428,7 +1428,7 @@ extern void _drbd_bm_set_bits(struct drbd_conf *mdev,
                 const unsigned long s, const unsigned long e);
 extern int  drbd_bm_test_bit(struct drbd_conf *mdev, unsigned long bitnr);
 extern int  drbd_bm_e_weight(struct drbd_conf *mdev, unsigned long enr);
-extern int  drbd_bm_write_sect(struct drbd_conf *mdev, unsigned long enr) __must_hold(local);
+extern int  drbd_bm_write_page(struct drbd_conf *mdev, unsigned int idx) __must_hold(local);
 extern int  drbd_bm_read(struct drbd_conf *mdev) __must_hold(local);
 extern int  drbd_bm_write(struct drbd_conf *mdev) __must_hold(local);
 extern unsigned long drbd_bm_ALe_set_all(struct drbd_conf *mdev,
@@ -1446,7 +1446,7 @@ extern int drbd_bm_rs_done(struct drbd_conf *mdev);
 /* for receive_bitmap */
 extern void drbd_bm_merge_lel(struct drbd_conf *mdev, size_t offset,
                 size_t number, unsigned long *buffer);
-/* for _drbd_send_bitmap and drbd_bm_write_sect */
+/* for _drbd_send_bitmap */
 extern void drbd_bm_get_lel(struct drbd_conf *mdev, size_t offset,
                 size_t number, unsigned long *buffer);
 
@@ -1641,7 +1641,6 @@ extern int __drbd_set_out_of_sync(struct drbd_conf *mdev, sector_t sector,
 #define drbd_set_out_of_sync(mdev, sector, size) \
         __drbd_set_out_of_sync(mdev, sector, size, __FILE__, __LINE__)
 extern void drbd_al_apply_to_bm(struct drbd_conf *mdev);
-extern void drbd_al_to_on_disk_bm(struct drbd_conf *mdev);
 extern void drbd_al_shrink(struct drbd_conf *mdev);
 
 
diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index 67fffad213ec..57ed7181742d 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c
@@ -1289,6 +1289,26 @@ static void abw_start_sync(struct drbd_conf *mdev, int rv)
         }
 }
 
+int drbd_bitmap_io_from_worker(struct drbd_conf *mdev, int (*io_fn)(struct drbd_conf *), char *why)
+{
+        int rv;
+
+        D_ASSERT(current == mdev->worker.task);
+
+        /* open coded non-blocking drbd_suspend_io(mdev); */
+        set_bit(SUSPEND_IO, &mdev->flags);
+        if (!is_susp(mdev->state))
+                D_ASSERT(atomic_read(&mdev->ap_bio_cnt) == 0);
+
+        drbd_bm_lock(mdev, why);
+        rv = io_fn(mdev);
+        drbd_bm_unlock(mdev);
+
+        drbd_resume_io(mdev);
+
+        return rv;
+}
+
 /**
  * after_state_ch() - Perform after state change actions that may sleep
  * @mdev:       DRBD device.
@@ -1404,7 +1424,12 @@ static void after_state_ch(struct drbd_conf *mdev, union drbd_state os,
 
                 /* D_DISKLESS Peer becomes secondary */
                 if (os.peer == R_PRIMARY && ns.peer == R_SECONDARY)
-                        drbd_al_to_on_disk_bm(mdev);
+                        drbd_bitmap_io_from_worker(mdev, &drbd_bm_write, "demote diskless peer");
+                put_ldev(mdev);
+        }
+
+        if (os.role == R_PRIMARY && ns.role == R_SECONDARY && get_ldev(mdev)) {
+                drbd_bitmap_io_from_worker(mdev, &drbd_bm_write, "demote");
                 put_ldev(mdev);
         }
 
diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index 2f0724982143..77dc022eaf6b 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -407,10 +407,8 @@ drbd_set_role(struct drbd_conf *mdev, enum drbd_role new_role, int force)
                 }
         }
 
-        if ((new_role == R_SECONDARY) && get_ldev(mdev)) {
-                drbd_al_to_on_disk_bm(mdev);
-                put_ldev(mdev);
-        }
+        /* writeout of activity log covered areas of the bitmap
+         * to stable storage done in after state change already */
 
         if (mdev->state.conn >= C_WF_REPORT_PARAMS) {
                 /* if this was forced, we should consider sync */
@@ -1174,7 +1172,10 @@ static int drbd_nl_disk_conf(struct drbd_conf *mdev, struct drbd_nl_cfg_req *nlp
 
         if (cp_discovered) {
                 drbd_al_apply_to_bm(mdev);
-                drbd_al_to_on_disk_bm(mdev);
+                if (drbd_bitmap_io(mdev, &drbd_bm_write, "crashed primary apply AL")) {
+                        retcode = ERR_IO_MD_DISK;
+                        goto force_diskless_dec;
+                }
         }
 
         if (_drbd_bm_total_weight(mdev) == drbd_bm_bits(mdev))
diff --git a/drivers/block/drbd/drbd_worker.c b/drivers/block/drbd/drbd_worker.c
index 9fe3e890da0f..d17f2ed777ce 100644
--- a/drivers/block/drbd/drbd_worker.c
+++ b/drivers/block/drbd/drbd_worker.c
@@ -907,10 +907,8 @@ out:
 
         drbd_md_sync(mdev);
 
-        if (test_and_clear_bit(WRITE_BM_AFTER_RESYNC, &mdev->flags)) {
-                dev_info(DEV, "Writing the whole bitmap\n");
-                drbd_queue_bitmap_io(mdev, &drbd_bm_write, NULL, "write from resync_finished");
-        }
+        dev_info(DEV, "Writing changed bitmap pages\n");
+        drbd_queue_bitmap_io(mdev, &drbd_bm_write, NULL, "write from resync_finished");
 
         if (khelper_cmd)
                 drbd_khelper(mdev, khelper_cmd);
@@ -1127,7 +1125,6 @@ void drbd_ov_oos_found(struct drbd_conf *mdev, sector_t sector, int size)
                 mdev->ov_last_oos_size = size>>9;
         }
         drbd_set_out_of_sync(mdev, sector, size);
-        set_bit(WRITE_BM_AFTER_RESYNC, &mdev->flags);
 }
 
 int w_e_end_ov_reply(struct drbd_conf *mdev, struct drbd_work *w, int cancel)