Re: [PATCH 2/2] ext4: Fix stale data exposure when read races with hole punch

Amir Goldstein
2019-06-03T16:34:03Z

On Mon, Jun 3, 2019 at 4:22 PM Jan Kara <jack@suse.cz> wrote:
>
> Hole puching currently evicts pages from page cache and then goes on to
> remove blocks from the inode. This happens under both i_mmap_sem and
> i_rwsem held exclusively which provides appropriate serialization with
> racing page faults. However there is currently nothing that prevents
> ordinary read(2) from racing with the hole punch and instantiating page
> cache page after hole punching has evicted page cache but before it has
> removed blocks from the inode. This page cache page will be mapping soon
> to be freed block and that can lead to returning stale data to userspace
> or even filesystem corruption.
>
> Fix the problem by protecting reads as well as readahead requests with
> i_mmap_sem.
>

So ->write_iter() does not take i_mmap_sem right?
and therefore mixed randrw workload is not expected to regress heavily
because of this change?

Did you test performance diff?
Here [1] I posted results of fio test that did x5 worse in xfs vs.
ext4, but I've
seen much worse cases.

Thanks,
Amir.

[1] https://lore.kernel.org/linux-fsdevel/CAOQ4uxhu=Qtme9RJ7uZXYXt0UE+=xD+OC4gQ9EYkDC1ap8Hizg@mail.gmail.com/