[PATCH 19/19] buffer: stop clearing BH_Uptodate when a write fails

[Chao Shi <coshi036@gmail.com>]

A buffer whose write failed still holds exactly the data the filesystem
asked to be written.  It is the disk that is out of date, not the buffer.
Clearing BH_Uptodate says the opposite, and callers act on it:

 - mark_buffer_dirty() has a WARN_ON_ONCE(!buffer_uptodate(bh)).  A
   filesystem that dirties the buffer again after a failed write - which is
   the normal way to retry - trips it.  That is the warning this series
   started from.

 - a buffer that is not up to date gets re-read from disk, which replaces
   the data the filesystem was trying to write with the stale on-disk copy,
   silently.

 - the window between the write completing and the buffer being marked not
   up to date is visible to anyone holding the folio lock, so the state is
   not even self consistent while it lasts.

BH_Write_EIO already records the failure, and by now every place in the
tree that needs to know about it tests that flag instead: the two core
helpers in this file, adfs, exfat, ext2, ext4, fat, gfs2, jbd2, ocfs2 and
omfs, converted one filesystem at a time in the preceding patches.  The
private completion handlers in jbd2 and ext4 fast commit were converted
along with their waiters.  Nothing is left that reads BH_Uptodate to find
out whether a write failed, so the clears can go.

Found by FuzzNvme.

Signed-off-by: Chao Shi <coshi036@gmail.com>
---
 fs/buffer.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index ac978d9090c2..0002f0736398 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -207,7 +207,6 @@ void bh_end_write(struct bio *bio)
 	} else {
 		buffer_io_error(bh, ", lost sync page write");
 		mark_buffer_write_io_error(bh);
-		clear_buffer_uptodate(bh);
 	}
 	unlock_buffer(bh);
 }
@@ -441,7 +440,6 @@ void bh_end_async_write(struct bio *bio)
 	} else {
 		buffer_io_error(bh, ", lost async page write");
 		mark_buffer_write_io_error(bh);
-		clear_buffer_uptodate(bh);
 	}
 
 	first = folio_buffers(folio);
-- 
2.43.0