linux-kernel - Re: [PATCH mm-nonmm-unstable v2 1/2] squashfs: fix page update race

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Date:   Thu, 29 Jun 2023 10:57:37 +0000
From:   Vincent Whitchurch <Vincent.Whitchurch@...s.com>
To:     Vincent Whitchurch <Vincent.Whitchurch@...s.com>,
        "phillip@...ashfs.org.uk" <phillip@...ashfs.org.uk>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "hch@....de" <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel <kernel@...s.com>
Subject: Re: [PATCH mm-nonmm-unstable v2 1/2] squashfs: fix page update race

On Fri, 2023-05-26 at 15:57 +0200, Vincent Whitchurch wrote:
> We only put the page into the cache after we've read it, so the
> PageUptodate() check should not be necessary.  In fact, it's actively
> harmful since the check could fail (since we used find_get_page() and
> not find_lock_page()) and we could end up submitting a page for I/O
> after it has been read and while it's actively being used, which could
> lead to corruption depending on what the block driver does with it.

It turns out that removing the PageUptodate() check entirely wasn't
correct.

While it's true that the squashfs code only puts the page into the cache
after it's been read as I wrote above, migration on the other hand
replaces the page in the mapping _before_ copying the contents over, so
a PageUptodate() check is still needed.

The original problem can be fixed by moving the PageUptodate() check to
squashfs_bio_read() and ignoring the cached page entirely if it's not up
to date.

I'll post a fix shortly.