Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <CAHk-=wh0_3y5s7-G74U0Pcjm7Y_yHB608NYrQSvgogVNBxsWSQ@mail.gmail.com>
 <YXBFqD9WVuU8awIv@arm.com> <CAHk-=wgv=KPZBJGnx_O5-7hhST8CL9BN4wJwtVuycjhv_1MmvQ@mail.gmail.com>
 <YXCbv5gdfEEtAYo8@arm.com> <CAHk-=wgP058PNY8eoWW=5uRMox-PuesDMrLsrCWPS+xXhzbQxQ@mail.gmail.com>
 <YXL9tRher7QVmq6N@arm.com> <CAHk-=wg4t2t1AaBDyMfOVhCCOiLLjCB5TFVgZcV4Pr8X2qptJw@mail.gmail.com>
 <CAHc6FU7BEfBJCpm8wC3P+8GTBcXxzDWcp6wAcgzQtuaJLHrqZA@mail.gmail.com>
 <YXhH0sBSyTyz5Eh2@arm.com> <CAHk-=wjWDsB-dDj+x4yr8h8f_VSkyB7MbgGqBzDRMNz125sZxw@mail.gmail.com>
 <YXmkvfL9B+4mQAIo@arm.com>
In-Reply-To: <YXmkvfL9B+4mQAIo@arm.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 27 Oct 2021 14:14:48 -0700
Message-ID: <CAHk-=wjQqi9cw1Guz6a8oBB0xiQNF_jtFzs3gW0k7+fKN-mB1g@mail.gmail.com>
Subject: Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, kvm-ppc@vger.kernel.org,
        linux-btrfs <linux-btrfs@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Oct 27, 2021 at 12:13 PM Catalin Marinas
<catalin.marinas@arm.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