linux-kernel - Re: [PATCH v2] mm: don't expose page to fast gup prematurely

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Date:   Wed, 25 Sep 2019 21:58:44 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Lance Roy <ldr709@...il.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Airlie <airlied@...hat.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Mel Gorman <mgorman@...e.de>, Jan Kara <jack@...e.cz>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Huang Ying <ying.huang@...el.com>,
        Aaron Lu <ziqian.lzq@...fin.com>,
        Omar Sandoval <osandov@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: don't expose page to fast gup prematurely

On Wed, Sep 25, 2019 at 03:17:50PM +0300, Kirill A. Shutemov wrote:
> 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?

We have the pages locked in those two places, so for try_to_unmap()
and the rest of page_vma_mapped_walk() users, they will block on
the page lock:
	CPU 1			CPU 2
	lock_page()
	set_pte_at()
	unlock_page()
				lock_page()
				try_to_unmap()
				  page_vma_mapped_walk()
				    pte_present() without holding ptl
				unlock_page()

For others that don't use page_vma_mapped_walk(), they should either
lock pages or grab ptl before checking pte_present().

AFAIK, the fast gup is the only one doesn't fall into the either
category.