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Date: Wed, 4 Jan 2012 15:16:32 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Hillf Danton <dhillf@gmail.com>
Cc: Michal Hocko <mhocko@suse.cz>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [PATCH] mm: hugetlb: undo change to page mapcount in fault
 handler
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On Fri, 23 Dec 2011 21:00:41 +0800
Hillf Danton <dhillf@gmail.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?