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

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Date:   Mon, 3 Jun 2019 19:33:50 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ext4 <linux-ext4@...r.kernel.org>, Ted Tso <tytso@....edu>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: Fix stale data exposure when read races with
 hole punch

On Mon, Jun 3, 2019 at 4:22 PM Jan Kara <jack@...e.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/