linux-kernel - Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

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Message-ID: <CAHk-=wjQqi9cw1Guz6a8oBB0xiQNF_jtFzs3gW0k7+fKN-mB1g@mail.gmail.com>
Date:   Wed, 27 Oct 2021 14:14:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Paul Mackerras <paulus@...abs.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com, kvm-ppc@...r.kernel.org,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

On Wed, Oct 27, 2021 at 12:13 PM Catalin Marinas
<catalin.marinas@....com> wrote:
>
> As an alternative, you mentioned earlier that a per-thread fault status
> was not feasible on x86 due to races. Was this only for the hw poison
> case? I think the uaccess is slightly different.

It's not x86-specific, it's very generic.

If we set some flag in the per-thread status, we'll need to be careful
about not overwriting it if we then have a subsequent NMI that _also_
takes a (completely unrelated) page fault - before we then read the
per-thread flag.

Think 'perf' and fetching backtraces etc.

Note that the NMI page fault can easily also be a pointer coloring
fault on arm64, for exactly the same reason that whatever original
copy_from_user() code was. So this is not a "oh, pointer coloring
faults are different". They have the same re-entrancy issue.

And both the "pagefault_disable" and "fault happens in interrupt
context" cases are also the exact same 'faulthandler_disabled()'
thing. So even at fault time they look very similar.

So we'd have to have some way to separate out only the one we care about.

               Linus