linux-kernel - Re: [PATCH] mm/gup: Let __get_user_pages_locked() return -EINTR for fatal signal

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Message-ID: <CAHk-=wjR=rtvm21=yP_1tqscXpOSEVpZaJ+oBAD8qU9ZKeZEWQ@mail.gmail.com>
Date:   Wed, 8 Apr 2020 09:20:00 -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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        syzbot+3be1a33f04dc782e9fd5@...kaller.appspotmail.com,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm/gup: Let __get_user_pages_locked() return -EINTR for
 fatal signal

On Wed, Apr 8, 2020 at 8:59 AM Peter Xu <peterx@...hat.com> wrote:
>
> __get_user_pages_locked() will return 0 instead of -EINTR after commit
> 4426e945df588 which added extra code to allow gup detect fatal signal
> faster.  Restore that behavior.

I've applied this, but it's worth noting that
__get_user_pages_locked() can still return 0 in other situations.

I realize that "I got zero pages" is a valid return value, but I do
wonder if we should make the rule be that a zero return value isn't
possible (return -EAGAIN or whatever if you doin't have the
EFAULT/EINTR conditions).

So that you'd always get either an error, or a successful number of pages.

The only case where __get_user_pages_locked() might return zero is if
you pass in a zero 'nr_pages', although I suspect even for that case
returning -EINVAL might be a better option.

Anyway, this is not a new issue, but I thought I'd mention it.

             Linus