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
Date: Fri, 13 Jan 2012 15:39:50 -0800 [thread overview]
Message-ID: <20120113153950.7426eee2.akpm@linux-foundation.org> (raw)
In-Reply-To: <CAJd=RBDOn22=CAFcEx9try8onsaHsweny_B1ZvnGJO-0h7eZAQ@mail.gmail.com>
On Wed, 11 Jan 2012 20:06:30 +0800
Hillf Danton <dhillf@gmail.com> wrote:
> On Thu, Jan 5, 2012 at 7:16 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
> > 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?
>
> Hi Andrew
>
> IIUC the two operations, vma_{needs, commit}_reservation, are folded in
> alloc_huge_page(), need to break the pair?
Looking at it again, it appears that the vma_needs_reservation() calls
are used to predict whether a subsequent COW attempt is going to fail.
If that's correct then things aren't as bad as I first thought.
However I suspect the code in hugetlb_no_page() is a bit racy: the
vma_needs_reservation() call should happen after we've taken
page_table_lock. As things stand, another thread could sneak in there
and steal the reservation which this thread thought was safe.
What do you think?
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next prev parent reply other threads:[~2012-01-13 23:39 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-22 13:36 Hillf Danton
2011-12-22 16:36 ` Michal Hocko
2011-12-23 13:00 ` Hillf Danton
2011-12-26 7:00 ` KAMEZAWA Hiroyuki
2012-01-04 23:16 ` Andrew Morton
2012-01-10 20:45 ` Andrew Morton
2012-01-11 12:06 ` Hillf Danton
2012-01-13 23:39 ` Andrew Morton [this message]
2012-01-14 5:27 ` Hillf Danton
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