dm bufio: prefetch

This patch introduces a new function dm_bufio_prefetch. It prefetches
the specified range of blocks into dm-bufio cache without waiting
for i/o completion.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

2 files changed, 90 insertions, 26 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index b6e58c7b6df5..cc06a1e52423 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -578,7 +578,7 @@ static void write_endio(struct bio *bio, int error)
         struct dm_buffer *b = container_of(bio, struct dm_buffer, bio);
 
         b->write_error = error;
-        if (error) {
+        if (unlikely(error)) {
                 struct dm_bufio_client *c = b->c;
                 (void)cmpxchg(&c->async_write_error, 0, error);
         }
@@ -697,13 +697,20 @@ static void __wait_for_free_buffer(struct dm_bufio_client *c)
         dm_bufio_lock(c);
 }
 
+enum new_flag {
+        NF_FRESH = 0,
+        NF_READ = 1,
+        NF_GET = 2,
+        NF_PREFETCH = 3
+};
+
 /*
  * Allocate a new buffer. If the allocation is not possible, wait until
  * some other thread frees a buffer.
  *
  * May drop the lock and regain it.
  */
-static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c)
+static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client *c, enum new_flag nf)
 {
         struct dm_buffer *b;
 
@@ -726,6 +733,9 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
                                 return b;
                 }
 
+                if (nf == NF_PREFETCH)
+                        return NULL;
+
                 if (!list_empty(&c->reserved_buffers)) {
                         b = list_entry(c->reserved_buffers.next,
                                        struct dm_buffer, lru_list);
@@ -743,9 +753,12 @@ static struct dm_buffer *__alloc_buffer_wait_no_callback(struct dm_bufio_client
         }
 }
 
-static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c)
+static struct dm_buffer *__alloc_buffer_wait(struct dm_bufio_client *c, enum new_flag nf)
 {
-        struct dm_buffer *b = __alloc_buffer_wait_no_callback(c);
+        struct dm_buffer *b = __alloc_buffer_wait_no_callback(c, nf);
+
+        if (!b)
+                return NULL;
 
         if (c->alloc_callback)
                 c->alloc_callback(b);
@@ -865,32 +878,23 @@ static struct dm_buffer *__find(struct dm_bufio_client *c, sector_t block)
  * Getting a buffer
  *--------------------------------------------------------------*/
 
-enum new_flag {
-        NF_FRESH = 0,
-        NF_READ = 1,
-        NF_GET = 2
-};
-
 static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
-                                     enum new_flag nf, struct dm_buffer **bp,
-                                     int *need_submit)
+                                     enum new_flag nf, int *need_submit)
 {
         struct dm_buffer *b, *new_b = NULL;
 
         *need_submit = 0;
 
         b = __find(c, block);
-        if (b) {
-                b->hold_count++;
-                __relink_lru(b, test_bit(B_DIRTY, &b->state) ||
-                             test_bit(B_WRITING, &b->state));
-                return b;
-        }
+        if (b)
+                goto found_buffer;
 
         if (nf == NF_GET)
                 return NULL;
 
-        new_b = __alloc_buffer_wait(c);
+        new_b = __alloc_buffer_wait(c, nf);
+        if (!new_b)
+                return NULL;
 
         /*
          * We've had a period where the mutex was unlocked, so need to
@@ -899,10 +903,7 @@ static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
         b = __find(c, block);
         if (b) {
                 __free_buffer_wake(new_b);
-                b->hold_count++;
-                __relink_lru(b, test_bit(B_DIRTY, &b->state) ||
-                             test_bit(B_WRITING, &b->state));
-                return b;
+                goto found_buffer;
         }
 
         __check_watermark(c);
@@ -922,6 +923,24 @@ static struct dm_buffer *__bufio_new(struct dm_bufio_client *c, sector_t block,
         *need_submit = 1;
 
         return b;
+
+found_buffer:
+        if (nf == NF_PREFETCH)
+                return NULL;
+        /*
+         * Note: it is essential that we don't wait for the buffer to be
+         * read if dm_bufio_get function is used. Both dm_bufio_get and
+         * dm_bufio_prefetch can be used in the driver request routine.
+         * If the user called both dm_bufio_prefetch and dm_bufio_get on
+         * the same buffer, it would deadlock if we waited.
+         */
+        if (nf == NF_GET && unlikely(test_bit(B_READING, &b->state)))
+                return NULL;
+
+        b->hold_count++;
+        __relink_lru(b, test_bit(B_DIRTY, &b->state) ||
+                     test_bit(B_WRITING, &b->state));
+        return b;
 }
 
 /*
@@ -956,10 +975,10 @@ static void *new_read(struct dm_bufio_client *c, sector_t block,
         struct dm_buffer *b;
 
         dm_bufio_lock(c);
-        b = __bufio_new(c, block, nf, bp, &need_submit);
+        b = __bufio_new(c, block, nf, &need_submit);
         dm_bufio_unlock(c);
 
-        if (!b || IS_ERR(b))
+        if (!b)
                 return b;
 
         if (need_submit)
@@ -1005,13 +1024,47 @@ void *dm_bufio_new(struct dm_bufio_client *c, sector_t block,
 }
 EXPORT_SYMBOL_GPL(dm_bufio_new);
 
+void dm_bufio_prefetch(struct dm_bufio_client *c,
+                       sector_t block, unsigned n_blocks)
+{
+        struct blk_plug plug;
+
+        blk_start_plug(&plug);
+        dm_bufio_lock(c);
+
+        for (; n_blocks--; block++) {
+                int need_submit;
+                struct dm_buffer *b;
+                b = __bufio_new(c, block, NF_PREFETCH, &need_submit);
+                if (unlikely(b != NULL)) {
+                        dm_bufio_unlock(c);
+
+                        if (need_submit)
+                                submit_io(b, READ, b->block, read_endio);
+                        dm_bufio_release(b);
+
+                        dm_bufio_cond_resched();
+
+                        if (!n_blocks)
+                                goto flush_plug;
+                        dm_bufio_lock(c);
+                }
+
+        }
+
+        dm_bufio_unlock(c);
+
+flush_plug:
+        blk_finish_plug(&plug);
+}
+EXPORT_SYMBOL_GPL(dm_bufio_prefetch);
+
 void dm_bufio_release(struct dm_buffer *b)
 {
         struct dm_bufio_client *c = b->c;
 
         dm_bufio_lock(c);
 
-        BUG_ON(test_bit(B_READING, &b->state));
         BUG_ON(!b->hold_count);
 
         b->hold_count--;
@@ -1024,6 +1077,7 @@ void dm_bufio_release(struct dm_buffer *b)
                  * invalid buffer.
                  */
                 if ((b->read_error || b->write_error) &&
+                    !test_bit(B_READING, &b->state) &&
                     !test_bit(B_WRITING, &b->state) &&
                     !test_bit(B_DIRTY, &b->state)) {
                         __unlink_buffer(b);
@@ -1041,6 +1095,8 @@ void dm_bufio_mark_buffer_dirty(struct dm_buffer *b)
 
         dm_bufio_lock(c);
 
+        BUG_ON(test_bit(B_READING, &b->state));
+
         if (!test_and_set_bit(B_DIRTY, &b->state))
                 __relink_lru(b, LIST_DIRTY);
 
diff --git a/drivers/md/dm-bufio.h b/drivers/md/dm-bufio.h
index 5c4c3a04e381..b142946a9e32 100644
--- a/drivers/md/dm-bufio.h
+++ b/drivers/md/dm-bufio.h
@@ -63,6 +63,14 @@ void *dm_bufio_new(struct dm_bufio_client *c, sector_t block,
                    struct dm_buffer **bp);
 
 /*
+ * Prefetch the specified blocks to the cache.
+ * The function starts to read the blocks and returns without waiting for
+ * I/O to finish.
+ */
+void dm_bufio_prefetch(struct dm_bufio_client *c,
+                       sector_t block, unsigned n_blocks);
+
+/*
  * Release a reference obtained with dm_bufio_{read,get,new}. The data
  * pointer and dm_buffer pointer is no longer valid after this call.
  */