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

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Date:   Wed, 20 Oct 2021 10:11:19 -1000
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 20, 2021 at 6:37 AM Catalin Marinas <catalin.marinas@....com> wrote:
>
> The atomic "add zero" trick isn't that simple for MTE since the arm64
> atomic or exclusive instructions run with kernel privileges and
> therefore with the kernel tag checking mode.

Are there any instructions that are useful for "probe_user_write()"
kind of thing? We could always just add that as an arch function, with
a fallback to using the futex "add zero" if the architecture doesn't
need anything special.

Although at least for MTE, I think the solution was to do a regular
read, and that checks the tag, and then we could use the gup machinery
for the writability checks.

                Linus