The syzbot has reported that it can hit the warning in
ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
reproducer creates a race between DIO IO completion and truncate
expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
inode state where i_disksize is already updated but i_size is not
updated yet. Since we are careful when setting up DIO write and consider
it extending (and thus performing the IO synchronously with i_rwsem held
exclusively) whenever it goes past either of i_size or i_disksize, we
can use the same test during IO completion without risking entering
ext4_handle_inode_extension() without i_rwsem held. This way we make it
obvious both i_size and i_disksize are large enough when we report DIO
completion without relying on unreliable WARN_ON.
Reported-by: [email protected]
Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
Signed-off-by: Jan Kara <[email protected]>
---
fs/ext4/file.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
Changes since v1:
* Expanded comment in ext4_inode_extension_cleanup()
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0166bb9ca160..6aa15dafc677 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -349,9 +349,10 @@ static void ext4_inode_extension_cleanup(struct inode *inode, ssize_t count)
return;
}
/*
- * If i_disksize got extended due to writeback of delalloc blocks while
- * the DIO was running we could fail to cleanup the orphan list in
- * ext4_handle_inode_extension(). Do it now.
+ * If i_disksize got extended either due to writeback of delalloc
+ * blocks or extending truncate while the DIO was running we could fail
+ * to cleanup the orphan list in ext4_handle_inode_extension(). Do it
+ * now.
*/
if (!list_empty(&EXT4_I(inode)->i_orphan) && inode->i_nlink) {
handle_t *handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
@@ -386,10 +387,11 @@ static int ext4_dio_write_end_io(struct kiocb *iocb, ssize_t size,
* blocks. But the code in ext4_iomap_alloc() is careful to use
* zeroed/unwritten extents if this is possible; thus we won't leave
* uninitialized blocks in a file even if we didn't succeed in writing
- * as much as we intended.
+ * as much as we intended. Also we can race with truncate or write
+ * expanding the file so we have to be a bit careful here.
*/
- WARN_ON_ONCE(i_size_read(inode) < READ_ONCE(EXT4_I(inode)->i_disksize));
- if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize))
+ if (pos + size <= READ_ONCE(EXT4_I(inode)->i_disksize) &&
+ pos + size <= i_size_read(inode))
return size;
return ext4_handle_inode_extension(inode, pos, size);
}
--
2.35.3
Jan Kara <[email protected]> writes:
> The syzbot has reported that it can hit the warning in
> ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
> reproducer creates a race between DIO IO completion and truncate
> expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
> inode state where i_disksize is already updated but i_size is not
> updated yet. Since we are careful when setting up DIO write and consider
> it extending (and thus performing the IO synchronously with i_rwsem held
> exclusively) whenever it goes past either of i_size or i_disksize, we
> can use the same test during IO completion without risking entering
> ext4_handle_inode_extension() without i_rwsem held. This way we make it
> obvious both i_size and i_disksize are large enough when we report DIO
> completion without relying on unreliable WARN_ON.
>
> Reported-by: [email protected]
> Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
> Signed-off-by: Jan Kara <[email protected]>
> ---
> fs/ext4/file.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> Changes since v1:
> * Expanded comment in ext4_inode_extension_cleanup()
Looks good to me. Please feel free to add -
Reviewed-by: Ritesh Harjani (IBM) <[email protected]>
On Thu, 30 Nov 2023 10:56:53 +0100, Jan Kara wrote:
> The syzbot has reported that it can hit the warning in
> ext4_dio_write_end_io() because i_size < i_disksize. Indeed the
> reproducer creates a race between DIO IO completion and truncate
> expanding the file and thus ext4_dio_write_end_io() sees an inconsistent
> inode state where i_disksize is already updated but i_size is not
> updated yet. Since we are careful when setting up DIO write and consider
> it extending (and thus performing the IO synchronously with i_rwsem held
> exclusively) whenever it goes past either of i_size or i_disksize, we
> can use the same test during IO completion without risking entering
> ext4_handle_inode_extension() without i_rwsem held. This way we make it
> obvious both i_size and i_disksize are large enough when we report DIO
> completion without relying on unreliable WARN_ON.
>
> [...]
Applied, thanks!
[1/1] ext4: Fix warning in ext4_dio_write_end_io()
commit: 619f75dae2cf117b1d07f27b046b9ffb071c4685
Best regards,
--
Theodore Ts'o <[email protected]>