linux-kernel - Re: [PATCH v5 02/25] mm: userfault: return VM_FAULT

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Message-ID: <CAHk-=wjxOz5RXpFTU=wSJg4Mjg1ugOBhBVppSTH6qjZPxpGOKg@mail.gmail.com>
Date:   Wed, 26 Jun 2019 09:59:58 +0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Jerome Glisse <jglisse@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v5 02/25] mm: userfault: return VM_FAULT_RETRY on signals

On Tue, Jun 25, 2019 at 1:31 PM Peter Xu <peterx@...hat.com> wrote:
>
> Yes that sounds reasonable to me, and that matches perfectly with
> TASK_INTERRUPTIBLE and TASK_KILLABLE.  The only thing that I am a bit
> uncertain is whether we should define FAULT_FLAG_INTERRUPTIBLE as a
> new bit or make it simply a combination of:
>
>   FAULT_FLAG_KILLABLE | FAULT_FLAG_USER

It needs to be a new bit, I think.

Some things could potentially care about the difference between "can I
abort this thing because the task will *die* and never see the end
result" and "can I abort this thing because it will be retried".

For a regular page fault, maybe FAULT_FLAG_INTERRUPTBLE will always be
set for the same things that set FAULT_FLAG_KILLABLE when it happens
from user mode, but at least conceptually I think they are different,
and it could make a difference for things like get_user_pages() or
similar.

Also, I actually don't think we should ever expose FAULT_FLAG_USER to
any fault handlers anyway. It has a very specific meaning for memory
cgroup handling, and no other fault handler should likely ever care
about "was this a user fault". So I'd actually prefer for people to
ignore and forget that hacky flag entirely, rather than give it subtle
semantic meaning together with KILLABLE.

[ Side note: this is the point where I may soon lose internet access,
so I'll probably not be able to participate in the discussion any more
for a while ]

             Linus