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Message-ID: <20190925121750.zxrt2zkc4g73h6cp@box>
Date: Wed, 25 Sep 2019 15:17:50 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...e.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...hat.com>,
Namhyung Kim <namhyung@...nel.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Vlastimil Babka <vbabka@...e.cz>,
Hugh Dickins <hughd@...gle.com>,
Jérôme Glisse <jglisse@...hat.com>,
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linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: don't expose page to fast gup prematurely
On Tue, Sep 24, 2019 at 04:05:50PM -0600, Yu Zhao wrote:
> On Tue, Sep 24, 2019 at 02:23:16PM +0300, Kirill A. Shutemov wrote:
> > On Sat, Sep 14, 2019 at 01:05:18AM -0600, Yu Zhao wrote:
> > > We don't want to expose page to fast gup running on a remote CPU
> > > before all local non-atomic ops on page flags are visible first.
> > >
> > > For anon page that isn't in swap cache, we need to make sure all
> > > prior non-atomic ops, especially __SetPageSwapBacked() in
> > > page_add_new_anon_rmap(), are order before set_pte_at() to prevent
> > > the following race:
> > >
> > > CPU 1 CPU1
> > > set_pte_at() get_user_pages_fast()
> > > page_add_new_anon_rmap() gup_pte_range()
> > > __SetPageSwapBacked() SetPageReferenced()
> >
> > Is there a particular codepath that has what you listed for CPU?
> > After quick look, I only saw that we page_add_new_anon_rmap() called
> > before set_pte_at().
>
> I think so. One in do_swap_page() and another in unuse_pte(). Both
> are on KSM paths. Am I referencing a stale copy of the source?
I *think* it is a bug. Setting a pte before adding the page to rmap may
lead to rmap (like try_to_unmap() or something) to miss the VMA.
Do I miss something?
--
Kirill A. Shutemov
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