[PATCH] add AOP_TRUNCATED_PAGE, prepend AOP_ to WRITEPAGE_ACTIVATE

readpage(), prepare_write(), and commit_write() callers are updated to
understand the special return code AOP_TRUNCATED_PAGE in the style of
writepage() and WRITEPAGE_ACTIVATE.  AOP_TRUNCATED_PAGE tells the caller that
the callee has unlocked the page and that the operation should be tried again
with a new page.  OCFS2 uses this to detect and work around a lock inversion in
its aop methods.  There should be no change in behaviour for methods that don't
return AOP_TRUNCATED_PAGE.

WRITEPAGE_ACTIVATE is also prepended with AOP_ for consistency and they are
made enums so that kerneldoc can be used to document their semantics.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

9 files changed, 113 insertions, 45 deletions

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 96c664af8d06..a452b13620a2 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -213,7 +213,7 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
         struct address_space_operations *aops = mapping->a_ops;
         pgoff_t index;
         unsigned offset, bv_offs;
-        int len, ret = 0;
+        int len, ret;
 
         down(&mapping->host->i_sem);
         index = pos >> PAGE_CACHE_SHIFT;
@@ -232,9 +232,15 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
                 page = grab_cache_page(mapping, index);
                 if (unlikely(!page))
                         goto fail;
-                if (unlikely(aops->prepare_write(file, page, offset,
-                                offset + size)))
+                ret = aops->prepare_write(file, page, offset,
+                                          offset + size);
+                if (unlikely(ret)) {
+                        if (ret == AOP_TRUNCATED_PAGE) {
+                                page_cache_release(page);
+                                continue;
+                        }
                         goto unlock;
+                }
                 transfer_result = lo_do_transfer(lo, WRITE, page, offset,
                                 bvec->bv_page, bv_offs, size, IV);
                 if (unlikely(transfer_result)) {
@@ -251,9 +257,15 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
                         kunmap_atomic(kaddr, KM_USER0);
                 }
                 flush_dcache_page(page);
-                if (unlikely(aops->commit_write(file, page, offset,
-                                offset + size)))
+                ret = aops->commit_write(file, page, offset,
+                                         offset + size);
+                if (unlikely(ret)) {
+                        if (ret == AOP_TRUNCATED_PAGE) {
+                                page_cache_release(page);
+                                continue;
+                        }
                         goto unlock;
+                }
                 if (unlikely(transfer_result))
                         goto unlock;
                 bv_offs += size;
@@ -264,6 +276,7 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
                 unlock_page(page);
                 page_cache_release(page);
         }
+        ret = 0;
 out:
         up(&mapping->host->i_sem);
         return ret;
diff --git a/drivers/block/rd.c b/drivers/block/rd.c
index 68c60a5bcdab..ffd6abd6d5a0 100644
--- a/drivers/block/rd.c
+++ b/drivers/block/rd.c
@@ -154,7 +154,7 @@ static int ramdisk_commit_write(struct file *file, struct page *page,
 
 /*
  * ->writepage to the the blockdev's mapping has to redirty the page so that the
- * VM doesn't go and steal it.  We return WRITEPAGE_ACTIVATE so that the VM
+ * VM doesn't go and steal it.  We return AOP_WRITEPAGE_ACTIVATE so that the VM
  * won't try to (pointlessly) write the page again for a while.
  *
  * Really, these pages should not be on the LRU at all.
@@ -165,7 +165,7 @@ static int ramdisk_writepage(struct page *page, struct writeback_control *wbc)
                 make_page_uptodate(page);
         SetPageDirty(page);
         if (wbc->for_reclaim)
-                return WRITEPAGE_ACTIVATE;
+                return AOP_WRITEPAGE_ACTIVATE;
         unlock_page(page);
         return 0;
 }
diff --git a/fs/mpage.c b/fs/mpage.c
index c5adcdddf3cc..f1d2d02bd4c8 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -721,7 +721,7 @@ retry:
                                                 &last_block_in_bio, &ret, wbc,
                                                 page->mapping->a_ops->writepage);
                         }
-                        if (unlikely(ret == WRITEPAGE_ACTIVATE))
+                        if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE))
                                 unlock_page(page);
                         if (ret || (--(wbc->nr_to_write) <= 0))
                                 done = 1;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index cc35b6ac778d..ed9a41a71e8b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -302,6 +302,37 @@ struct iattr {
  */
 #include <linux/quota.h>
 
+/** 
+ * enum positive_aop_returns - aop return codes with specific semantics
+ *
+ * @AOP_WRITEPAGE_ACTIVATE: Informs the caller that page writeback has
+ *                          completed, that the page is still locked, and
+ *                          should be considered active.  The VM uses this hint
+ *                          to return the page to the active list -- it won't
+ *                          be a candidate for writeback again in the near
+ *                          future.  Other callers must be careful to unlock
+ *                          the page if they get this return.  Returned by
+ *                          writepage(); 
+ *
+ * @AOP_TRUNCATED_PAGE: The AOP method that was handed a locked page has
+ *                      unlocked it and the page might have been truncated.
+ *                      The caller should back up to acquiring a new page and
+ *                      trying again.  The aop will be taking reasonable
+ *                      precautions not to livelock.  If the caller held a page
+ *                      reference, it should drop it before retrying.  Returned
+ *                      by readpage(), prepare_write(), and commit_write().
+ *
+ * address_space_operation functions return these large constants to indicate
+ * special semantics to the caller.  These are much larger than the bytes in a
+ * page to allow for functions that return the number of bytes operated on in a
+ * given page.
+ */
+
+enum positive_aop_returns {
+        AOP_WRITEPAGE_ACTIVATE  = 0x80000,
+        AOP_TRUNCATED_PAGE      = 0x80001,
+};
+
 /*
  * oh the beauties of C type declarations.
  */
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 343d883d69c5..64a36ba43b2f 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -60,12 +60,6 @@ struct writeback_control {
 };
 
 /*
- * ->writepage() return values (make these much larger than a pagesize, in
- * case some fs is returning number-of-bytes-written from writepage)
- */
-#define WRITEPAGE_ACTIVATE      0x80000 /* IO was not started: activate page */
-
-/*
  * fs/fs-writeback.c
  */     
 void writeback_inodes(struct writeback_control *wbc);
diff --git a/mm/filemap.c b/mm/filemap.c
index 33a28bfde158..6e1d08a2b8b9 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -831,8 +831,13 @@ readpage:
                 /* Start the actual read. The read will unlock the page. */
                 error = mapping->a_ops->readpage(filp, page);
 
-                if (unlikely(error))
+                if (unlikely(error)) {
+                        if (error == AOP_TRUNCATED_PAGE) {
+                                page_cache_release(page);
+                                goto find_page;
+                        }
                         goto readpage_error;
+                }
 
                 if (!PageUptodate(page)) {
                         lock_page(page);
@@ -1152,26 +1157,24 @@ static int fastcall page_cache_read(struct file * file, unsigned long offset)
 {
         struct address_space *mapping = file->f_mapping;
         struct page *page; 
-        int error;
+        int ret;
 
-        page = page_cache_alloc_cold(mapping);
-        if (!page)
-                return -ENOMEM;
+        do {
+                page = page_cache_alloc_cold(mapping);
+                if (!page)
+                        return -ENOMEM;
+
+                ret = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
+                if (ret == 0)
+                        ret = mapping->a_ops->readpage(file, page);
+                else if (ret == -EEXIST)
+                        ret = 0; /* losing race to add is OK */
 
-        error = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
-        if (!error) {
-                error = mapping->a_ops->readpage(file, page);
                 page_cache_release(page);
-                return error;
-        }
 
-        /*
-         * We arrive here in the unlikely event that someone 
-         * raced with us and added our page to the cache first
-         * or we are out of memory for radix-tree nodes.
-         */
-        page_cache_release(page);
-        return error == -EEXIST ? 0 : error;
+        } while (ret == AOP_TRUNCATED_PAGE);
+                
+        return ret;
 }
 
 #define MMAP_LOTSAMISS  (100)
@@ -1331,10 +1334,14 @@ page_not_uptodate:
                 goto success;
         }
 
-        if (!mapping->a_ops->readpage(file, page)) {
+        error = mapping->a_ops->readpage(file, page);
+        if (!error) {
                 wait_on_page_locked(page);
                 if (PageUptodate(page))
                         goto success;
+        } else if (error == AOP_TRUNCATED_PAGE) {
+                page_cache_release(page);
+                goto retry_find;
         }
 
         /*
@@ -1358,10 +1365,14 @@ page_not_uptodate:
                 goto success;
         }
         ClearPageError(page);
-        if (!mapping->a_ops->readpage(file, page)) {
+        error = mapping->a_ops->readpage(file, page);
+        if (!error) {
                 wait_on_page_locked(page);
                 if (PageUptodate(page))
                         goto success;
+        } else if (error == AOP_TRUNCATED_PAGE) {
+                page_cache_release(page);
+                goto retry_find;
         }
 
         /*
@@ -1444,10 +1455,14 @@ page_not_uptodate:
                 goto success;
         }
 
-        if (!mapping->a_ops->readpage(file, page)) {
+        error = mapping->a_ops->readpage(file, page);
+        if (!error) {
                 wait_on_page_locked(page);
                 if (PageUptodate(page))
                         goto success;
+        } else if (error == AOP_TRUNCATED_PAGE) {
+                page_cache_release(page);
+                goto retry_find;
         }
 
         /*
@@ -1470,10 +1485,14 @@ page_not_uptodate:
         }
 
         ClearPageError(page);
-        if (!mapping->a_ops->readpage(file, page)) {
+        error = mapping->a_ops->readpage(file, page);
+        if (!error) {
                 wait_on_page_locked(page);
                 if (PageUptodate(page))
                         goto success;
+        } else if (error == AOP_TRUNCATED_PAGE) {
+                page_cache_release(page);
+                goto retry_find;
         }
 
         /*
@@ -1934,12 +1953,16 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
                 status = a_ops->prepare_write(file, page, offset, offset+bytes);
                 if (unlikely(status)) {
                         loff_t isize = i_size_read(inode);
+
+                        if (status != AOP_TRUNCATED_PAGE)
+                                unlock_page(page);
+                        page_cache_release(page);
+                        if (status == AOP_TRUNCATED_PAGE)
+                                continue;
                         /*
                          * prepare_write() may have instantiated a few blocks
                          * outside i_size.  Trim these off again.
                          */
-                        unlock_page(page);
-                        page_cache_release(page);
                         if (pos + bytes > isize)
                                 vmtruncate(inode, isize);
                         break;
@@ -1952,6 +1975,10 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
                                                 cur_iov, iov_base, bytes);
                 flush_dcache_page(page);
                 status = a_ops->commit_write(file, page, offset, offset+bytes);
+                if (status == AOP_TRUNCATED_PAGE) {
+                        page_cache_release(page);
+                        continue;
+                }
                 if (likely(copied > 0)) {
                         if (!status)
                                 status = copied;
diff --git a/mm/readahead.c b/mm/readahead.c
index 72e7adbb87c7..8d6eeaaa6296 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -158,7 +158,7 @@ static int read_pages(struct address_space *mapping, struct file *filp,
 {
         unsigned page_idx;
         struct pagevec lru_pvec;
-        int ret = 0;
+        int ret;
 
         if (mapping->a_ops->readpages) {
                 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
@@ -171,14 +171,17 @@ static int read_pages(struct address_space *mapping, struct file *filp,
                 list_del(&page->lru);
                 if (!add_to_page_cache(page, mapping,
                                         page->index, GFP_KERNEL)) {
-                        mapping->a_ops->readpage(filp, page);
-                        if (!pagevec_add(&lru_pvec, page))
-                                __pagevec_lru_add(&lru_pvec);
-                } else {
-                        page_cache_release(page);
+                        ret = mapping->a_ops->readpage(filp, page);
+                        if (ret != AOP_TRUNCATED_PAGE) {
+                                if (!pagevec_add(&lru_pvec, page))
+                                        __pagevec_lru_add(&lru_pvec);
+                                continue;
+                        } /* else fall through to release */
                 }
+                page_cache_release(page);
         }
         pagevec_lru_add(&lru_pvec);
+        ret = 0;
 out:
         return ret;
 }
diff --git a/mm/shmem.c b/mm/shmem.c
index dc25565a61e9..d9fc277940da 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -855,7 +855,7 @@ unlock:
         swap_free(swap);
 redirty:
         set_page_dirty(page);
-        return WRITEPAGE_ACTIVATE;      /* Return with the page locked */
+        return AOP_WRITEPAGE_ACTIVATE;  /* Return with the page locked */
 }
 
 #ifdef CONFIG_NUMA
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b0cd81c32de6..795a050fe471 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -367,7 +367,7 @@ static pageout_t pageout(struct page *page, struct address_space *mapping)
                 res = mapping->a_ops->writepage(page, &wbc);
                 if (res < 0)
                         handle_write_error(mapping, page, res);
-                if (res == WRITEPAGE_ACTIVATE) {
+                if (res == AOP_WRITEPAGE_ACTIVATE) {
                         ClearPageReclaim(page);
                         return PAGE_ACTIVATE;
                 }