linux-kernel - Re: [PATCH] mm: hugetlb: undo change to page mapcount in fault handler

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Message-ID: <CAJd=RBDOn22=CAFcEx9try8onsaHsweny_B1ZvnGJO-0h7eZAQ@mail.gmail.com>
Date:	Wed, 11 Jan 2012 20:06:30 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] mm: hugetlb: undo change to page mapcount in fault handler

On Thu, Jan 5, 2012 at 7:16 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Fri, 23 Dec 2011 21:00:41 +0800
> Hillf Danton <dhillf@...il.com> wrote:
>
>> Page mapcount should be updated only if we are sure that the page ends
>> up in the page table otherwise we would leak if we couldn't COW due to
>> reservations or if idx is out of bounds.
>
> It would be much nicer if we could run vma_needs_reservation() before
> even looking up or allocating the page.
>
> And afaict the interface is set up to do that: you run
> vma_needs_reservation() before allocating the page and then
> vma_commit_reservation() afterwards.
>
> But hugetlb_no_page() and hugetlb_fault() appear to have forgotten to
> run vma_commit_reservation() altogether.  Why isn't this as busted as
> it appears to be?

Hi Andrew

IIUC the two operations, vma_{needs, commit}_reservation, are folded in
alloc_huge_page(), need to break the pair?

Thanks
Hillf
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