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

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Message-ID: <20200408171023.GC18914@dhcp22.suse.cz>
Date:   Wed, 8 Apr 2020 19:13:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, 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
Subject: Re: [PATCH] mm/gup: Let __get_user_pages_locked() return -EINTR for
 fatal signal

On Wed 08-04-20 09:20:00, Linus Torvalds wrote:
> 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.

Yeah, that makes sense to me. And get_user_pages_locked documentation
would benefit from an actual documentation as well. What we have right
now is more suited to the lower level (__get_user_pages_locked) and have
a high level user oriented documentation.
-- 
Michal Hocko
SUSE Labs