linux-kernel - Re: [PATCH 07/25] mm/csky: Use mm_fault

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Date:   Wed, 17 Jun 2020 10:53:23 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Guo Ren <guoren@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-csky@...r.kernel.org
Subject: Re: [PATCH 07/25] mm/csky: Use mm_fault_accounting()

On Wed, Jun 17, 2020 at 8:49 AM Peter Xu <peterx@...hat.com> wrote:
>
> I don't think it's a must, but mmap_sem should not be required at least by
> observing current code.  E.g., do_user_addr_fault() of x86 does the accounting
> without mmap_sem even before this series.

All the accounting should be per-thread and not need any locking.

Which is why a remote GUP should never account to the remote mm - not
only isn't there an unambiguous thread to account to (an mm can share
many threads), but it would require locking not just for the remote
update, but for all normal page faults.

                 Linus