buffer: fix unintended successful return

If try_to_free_buffers() succeeded and then folio_alloc_buffers() failed,
grow_dev_folio() would return success.  This would be incorrect; memory
allocation failure is supposed to result in a failure.  It's a harmless
bug; the caller will simply go around the loop one more time and
grow_dev_folio() will correctly return a failure that time.  But it was an
unintended change and looks like a more serious bug than it is.

While I'm in here, improve the commentary about why we return success even
though we failed.

Link: https://lkml.kernel.org/r/20240101093848.2017115-1-willy@infradead.org
Fixes: 6d840a18773f ("buffer: return bool from grow_dev_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/buffer.c b/fs/buffer.c
index 19548369bc6cab3216e7a8eb3e328d4007297825..5c29850e4781a4def78101cdaf5a03ed19dcbf9f 100644 (file)
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1028,8 +1028,8 @@ static sector_t folio_init_buffers(struct folio *folio,
  *
  * This is used purely for blockdev mappings.
  *
- * Returns false if we have a 'permanent' failure.  Returns true if
- * we succeeded, or the caller should retry.
+ * Returns false if we have a failure which cannot be cured by retrying
+ * without sleeping.  Returns true if we succeeded, or the caller should retry.
  */
 static bool grow_dev_folio(struct block_device *bdev, sector_t block,
                pgoff_t index, unsigned size, gfp_t gfp)
@@ -1051,10 +1051,17 @@ static bool grow_dev_folio(struct block_device *bdev, sector_t block,
                        goto unlock;
                }
 
-               /* Caller should retry if this call fails */
-               end_block = ~0ULL;
-               if (!try_to_free_buffers(folio))
+               /*
+                * Retrying may succeed; for example the folio may finish
+                * writeback, or buffers may be cleaned.  This should not
+                * happen very often; maybe we have old buffers attached to
+                * this blockdev's page cache and we're trying to change
+                * the block size?
+                */
+               if (!try_to_free_buffers(folio)) {
+                       end_block = ~0ULL;
                        goto unlock;
+               }
        }
 
        bh = folio_alloc_buffers(folio, size, gfp | __GFP_ACCOUNT);