From: Yu Zhao <yuzhao@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: don't expose page to fast gup before it's ready
Date: Tue, 14 May 2019 17:07:51 -0600 [thread overview]
Message-ID: <20190514230751.GA70050@google.com> (raw)
In-Reply-To: <20190514142527.356cb071155cd1077536f3da@linux-foundation.org>
On Tue, May 14, 2019 at 02:25:27PM -0700, Andrew Morton wrote:
> On Tue, 9 Jan 2018 02:10:50 -0800 Yu Zhao <yuzhao@google.com> wrote:
>
> > > Also what prevents reordering here? There do not seem to be any barriers
> > > to prevent __SetPageSwapBacked leak after set_pte_at with your patch.
> >
> > I assumed mem_cgroup_commit_charge() acted as full barrier. Since you
> > explicitly asked the question, I realized my assumption doesn't hold
> > when memcg is disabled. So we do need something to prevent reordering
> > in my patch. And it brings up the question whether we want to add more
> > barrier to other places that call page_add_new_anon_rmap() and
> > set_pte_at().
>
> Is a new version of this patch planned?
Sorry for the late reply. The last time I tried, I didn't come up
with a better fix because:
1) as Michal pointed out, we need to make sure the fast gup sees
all changes made before set_pte_at();
2) pairing smp_wmb() in set_pte/pmd_at() with smp_rmb() in gup
seems the best way to prevent any potential ordering related
problems in the future;
3) but this slows down the paths that don't require the smp_mwb()
unnecessarily.
I didn't give it further thought because the problem doesn't seem
fatal at the time. Now the fast gup has changed and the problem is
serious:
CPU 1 CPU 1
set_pte_at get_user_pages_fast
page_add_new_anon_rmap gup_pte_range
__SetPageSwapBacked (fetch)
try_get_compound_head
page_ref_add_unless
__SetPageSwapBacked (store)
Or the similar problem could happen to __do_huge_pmd_anonymous_page(),
for the reason of missing smp_wmb() between the non-atomic bit op
and set_pmd_at().
We could simply replace __SetPageSwapBacked() with its atomic
version. But 2) seems more preferable to me because it addresses
my original problem:
> > I didn't observe the race directly. But I did get few crashes when
> > trying to access mem_cgroup of pages returned by get_user_pages_fast().
> > Those page were charged and they showed valid mem_cgroup in kdumps.
> > So this led me to think the problem came from premature set_pte_at().
Thoughts? Thanks.
next prev parent reply other threads:[~2019-05-14 23:07 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-08 22:56 [PATCH] mm: don't expose page to fast gup before it's ready Yu Zhao
2018-01-09 8:46 ` Michal Hocko
2018-01-09 10:10 ` Yu Zhao
2018-01-31 23:07 ` Andrew Morton
2019-05-14 21:25 ` Andrew Morton
2019-05-14 23:07 ` Yu Zhao [this message]
2019-09-14 7:05 ` [PATCH v2] mm: don't expose page to fast gup prematurely Yu Zhao
2019-09-24 11:23 ` Kirill A. Shutemov
2019-09-24 22:05 ` Yu Zhao
2019-09-25 12:17 ` Kirill A. Shutemov
2019-09-26 3:58 ` Yu Zhao
2019-09-24 23:24 ` [PATCH v3 1/4] mm: remove unnecessary smp_wmb() in collapse_huge_page() Yu Zhao
2019-09-24 23:24 ` [PATCH v3 2/4] mm: don't expose hugetlb page to fast gup prematurely Yu Zhao
2019-09-24 23:24 ` [PATCH v3 3/4] mm: don't expose non-hugetlb " Yu Zhao
2019-09-25 8:25 ` Peter Zijlstra
2019-09-25 22:26 ` Yu Zhao
2019-09-26 10:20 ` Kirill A. Shutemov
2019-09-27 3:26 ` John Hubbard
2019-09-27 12:33 ` Michal Hocko
[not found] ` <20190927050648.GA92494@google.com>
[not found] ` <712513fe-f064-c965-d165-80d43cfc606f@nvidia.com>
2019-10-02 0:00 ` Yu Zhao
2019-09-24 23:24 ` [PATCH v3 4/4] mm: remove unnecessary smp_wmb() in __SetPageUptodate() Yu Zhao
2019-09-24 23:50 ` Matthew Wilcox
2019-09-25 22:03 ` Yu Zhao
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