linux-kernel - Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and get_user_pages

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Date:	Thu, 2 Oct 2014 14:40:43 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
Cc:	Peter Feiner <pfeiner@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gleb Natapov <gleb@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Paul Cassella <cassella@...y.com>,
	Hugh Dickins <hughd@...gle.com>,
	"\\Dr. David Alan Gilbert\\" <dgilbert@...hat.com>
Subject: Re: [PATCH 2/4] mm: gup: add get_user_pages_locked and
 get_user_pages_unlocked

On Wed, Oct 01, 2014 at 10:06:27AM -0700, Andres Lagar-Cavilla wrote:
> On Wed, Oct 1, 2014 at 8:51 AM, Peter Feiner <pfeiner@...gle.com> wrote:
> > On Wed, Oct 01, 2014 at 10:56:35AM +0200, Andrea Arcangeli wrote:
> >> +             /* VM_FAULT_RETRY cannot return errors */
> >> +             if (!*locked) {
> >> +                     BUG_ON(ret < 0);
> >> +                     BUG_ON(nr_pages == 1 && ret);
> >
> > If I understand correctly, this second BUG_ON is asserting that when
> > __get_user_pages is asked for a single page and it is successfully gets the
> > page, then it shouldn't have dropped the mmap_sem. If that's the case, then
> > you could generalize this assertion to
> >
> >                         BUG_ON(nr_pages == ret);

Agreed.

> 
> Even more strict:
>      BUG_ON(ret >= nr_pages);

Agreed too, plus this should be quicker than my weaker check.

Maybe some BUG_ON can be deleted later or converted to VM_BUG_ON, but
initially I feel safer with the BUG_ON considering that is a slow
path.

> Reviewed-by: Andres Lagar-Cavilla <andreslc@...gle.com>

Thanks!
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