On Thu 23-11-23 12:37:03, Ritesh Harjani wrote:
> Jan Kara <jack@xxxxxxx> 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.
>
> Does it make sense to add this in ext4_handle_inode_extension()?
> WARN_ON_ONCE(!inode_is_locked(inode));
> Ohk, we already have "lockdep_assert_held_write(&inode->i_rwsem)" so
> hopefully it can catch via lockdep.
Exactly.
> So, IIUC, the WARN happened when we were doing a non-extending
> AIO-DIO write which was racing with truncate trying to expand the file
> size. Because only then the DIO completion will not have i_rwsem held
> which can race with truncate. Truncate since it is expanding the file
> size, will not use inode_dio_wait() (since no block allocations).
>
> Is this understanding correct?
Yes, correct.
Honza
>
> >
> > Reported-by: syzbot+47479b71cdfc78f56d30@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 91562895f803 ("ext4: properly sync file size update after O_SYNC direct IO")
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> > fs/ext4/file.c | 7 ++++---
> > 1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> > index 0166bb9ca160..ba497aabdd1e 100644
> > --- a/fs/ext4/file.c
> > +++ b/fs/ext4/file.c
> > @@ -386,10 +386,11 @@ 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 <jack@xxxxxxxx>
SUSE Labs, CR
