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: <20211019134204.3382645-1-agruenba@redhat.com> <CAHk-=wh0_3y5s7-G74U0Pcjm7Y_yHB608NYrQSvgogVNBxsWSQ@mail.gmail.com>
 <YXBFqD9WVuU8awIv@arm.com>
In-Reply-To: <YXBFqD9WVuU8awIv@arm.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 20 Oct 2021 10:11:19 -1000
Message-ID: <CAHk-=wgv=KPZBJGnx_O5-7hhST8CL9BN4wJwtVuycjhv_1MmvQ@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 20, 2021 at 6:37 AM Catalin Marinas <catalin.marinas@arm.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