Ext4 needs to serialize unaligned direct AIO because the zeroing of
partial blocks of two competing unaligned AIOs can result in data
corruption.
However it decides not to serialize if the potentially unaligned aio is
past i_size with the rationale that no pending writes are possible past
i_size. Unfortunately if the i_size is not block aligned and the second
unaligned write lands past i_size, but still into the same block, it has
the potential of corrupting the previous unaligned write to the same
block.
This is (very simplified) reproducer from Frank
// 41472 = (10 * 4096) + 512
// 37376 = 41472 - 4096
ftruncate(fd, 41472);
io_prep_pwrite(iocbs[0], fd, buf[0], 4096, 37376);
io_prep_pwrite(iocbs[1], fd, buf[1], 4096, 41472);
io_submit(io_ctx, 1, &iocbs[1]);
io_submit(io_ctx, 1, &iocbs[2]);
io_getevents(io_ctx, 2, 2, events, NULL);
Without this patch the 512B range from 40960 up to the start of the
second unaligned write (41472) is going to be zeroed overwriting the data
written by the first write. This is a data corruption.
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
00009200 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
*
0000a000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0000a200 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
With this patch the data corruption is avoided because we will recognize
the unaligned_aio and wait for the unwritten extent conversion.
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
00009200 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
*
0000a200 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
*
0000b200
Reported-by: Frank Sorenson <fsorenso@xxxxxxxxxx>
Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
Fixes: e9e3bcecf44c ("ext4: serialize unaligned asynchronous DIO")
Cc: <stable@xxxxxxxxxxxxxxx>
---
fs/ext4/file.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 69d65d49837b..98ec11f69cd4 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -125,7 +125,7 @@ ext4_unaligned_aio(struct inode *inode, struct iov_iter *from, loff_t pos)
struct super_block *sb = inode->i_sb;
int blockmask = sb->s_blocksize - 1;
- if (pos >= i_size_read(inode))
+ if (pos >= ALIGN(i_size_read(inode), sb->s_blocksize))
return 0;
if ((pos | iov_iter_alignment(from)) & blockmask)
--
2.20.1
