Date:   Wed, 24 Nov 2021 13:36:00 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Will Deacon <will@kernel.org>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 0/3] Avoid live-lock in fault-in+uaccess loops with
 sub-page faults
Message-Id: <20211124133600.94f0b9a6c611ee663c9a8d6d@linux-foundation.org>
In-Reply-To: <20211124192024.2408218-1-catalin.marinas@arm.com>
References: <20211124192024.2408218-1-catalin.marinas@arm.com>
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On Wed, 24 Nov 2021 19:20:21 +0000 Catalin Marinas <catalin.marinas@arm.com> wrote:

> Hi,
> 
> There are a few places in the filesystem layer where a uaccess is
> performed in a loop with page faults disabled, together with a
> fault_in_*() call to pre-fault the pages. On architectures like arm64
> with MTE (memory tagging extensions) or SPARC ADI, even if the
> fault_in_*() succeeded, the uaccess can still fault indefinitely.
> 
> In general this is not an issue since such code restarts the
> fault_in_*() from where the uaccess failed, therefore guaranteeing
> forward progress. The btrfs search_ioctl(), however, rewinds the
> fault_in_*() position and it can live-lock. This was reported by Al
> here:

Btrfs livelock on some-of-arm sounds fairly serious.  Should we
backport this?  If so, a48b73eca4ce ("btrfs: fix potential deadlock in
the search ioctl") appears to be a suitable Fixes: target?