linux-kernel - Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

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Date:   Mon, 23 Sep 2019 11:03:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <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>,
        Marty McFadden <mcfadden8@...l.gov>, Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        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 v4 05/10] mm: Return faster for non-fatal signals in user
 mode faults

On Sun, Sep 22, 2019 at 9:26 PM Peter Xu <peterx@...hat.com> wrote:
>
> This patch is a preparation of removing that special path by allowing
> the page fault to return even faster if we were interrupted by a
> non-fatal signal during a user-mode page fault handling routine.

So I really wish saome other vm person would also review these things,
but looking over this series once more, this is the patch I probably
like the least.

And the reason I like it the least is that I have a hard time
explaining to myself what the code does and why, and why it's so full
of this pattern:

> -       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> +       if ((fault & VM_FAULT_RETRY) &&
> +           fault_should_check_signal(user_mode(regs)))
>                 return;

which isn't all that pretty.

Why isn't this just

  static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
  }

and then most of the users would be something like

        if (fault_signal_pending(fault, regs))
                return;

and the exceptions could do their own thing.

Now the code is prettier and more understandable, I feel.

And if something doesn't follow this pattern, maybe it either _should_
follow that pattern or it should just not use the helper but explain
why it has an unusual pattern.

             Linus