On Thu, Feb 20, 2020 at 10:15:48AM +0100, Jan Kara wrote: > On Wed 19-02-20 10:30:47, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > If EXT4_EXTENTS_FL is set on an inode while ext4_writepages() is running > > on it, the following warning in ext4_add_complete_io() can be hit: > > > > WARNING: CPU: 1 PID: 0 at fs/ext4/page-io.c:234 ext4_put_io_end_defer+0xf0/0x120 > > > > Here's a minimal reproducer (not 100% reliable) (root isn't required): > > > > while true; do > > sync > > done & > > while true; do > > rm -f file > > touch file > > chattr -e file > > echo X >> file > > chattr +e file > > done > > > > The problem is that in ext4_writepages(), ext4_should_dioread_nolock() > > (which only returns true on extent-based files) is checked once to set > > the number of reserved journal credits, and also again later to select > > the flags for ext4_map_blocks() and copy the reserved journal handle to > > ext4_io_end::handle. But if EXT4_EXTENTS_FL is being concurrently set, > > the first check can see dioread_nolock disabled while the later one can > > see it enabled, causing the reserved handle to unexpectedly be NULL. > > > > Since changing EXT4_EXTENTS_FL is uncommon, and there may be other races > > related to doing so as well, fix this by synchronizing changing > > EXT4_EXTENTS_FL with ext4_writepages() via the existing > > s_writepages_rwsem (previously called s_journal_flag_rwsem). > > > > This was originally reported by syzbot without a reproducer at > > https://syzkaller.appspot.com/bug?extid=2202a584a00fffd19fbf, > > but now that dioread_nolock is the default I also started seeing this > > when running syzkaller locally. > > > > Reported-by: syzbot+2202a584a00fffd19fbf@xxxxxxxxxxxxxxxxxxxxxxxxx > > Fixes: 6b523df4fb5a ("ext4: use transaction reservation for extent conversion in ext4_end_io") > > Cc: stable@xxxxxxxxxx > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > > The patch looks good to me. You can add: > > Reviewed-by: Jan Kara <jack@xxxxxxx> Thanks, applied. - Ted
