[PATCH] buffer: Fix unintended successful return

[PATCH] buffer: Fix unintended successful return

[Matthew Wilcox (Oracle)]

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.

Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/buffer.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index d5ce6b29c893..d3bcf601d3e5 100644
--- 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);
-- 
2.43.0

Re: [PATCH] buffer: Fix unintended successful return

[Andrew Morton]

On Mon,  1 Jan 2024 09:38:48 +0000 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> 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.
> 
> Reported-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Cool.  I'll add

Fixes: 6d840a18773f ("buffer: return bool from grow_dev_folio()")