linux-kernel - Re: [PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup prematurely

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Date:   Wed, 25 Sep 2019 16:26:54 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>,
        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>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        David Hildenbrand <david@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Anthony Yznaga <anthony.yznaga@...cle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup
 prematurely

On Wed, Sep 25, 2019 at 10:25:30AM +0200, Peter Zijlstra wrote:
> On Tue, Sep 24, 2019 at 05:24:58PM -0600, Yu Zhao wrote:
> > We don't want to expose a non-hugetlb page to the fast gup running
> > on a remote CPU before all local non-atomic ops on the page flags
> > are visible first.
> > 
> > For an 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 ordered 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()
> > 
> > This demonstrates a non-fatal scenario. Though haven't been directly
> > observed, the fatal ones can exist, e.g., PG_lock set by fast gup
> > caller and then overwritten by __SetPageSwapBacked().
> > 
> > For an anon page that is already in swap cache or a file page, we
> > don't need smp_wmb() before set_pte_at() because adding to swap or
> > file cach serves as a valid write barrier. Using non-atomic ops
> > thereafter is a bug, obviously.
> > 
> > smp_wmb() is added following 11 of total 12 page_add_new_anon_rmap()
> > call sites, with the only exception being
> > do_huge_pmd_wp_page_fallback() because of an existing smp_wmb().
> > 
> 
> I'm thinking this patch make stuff rather fragile.. Should we instead
> stick the barrier in set_p*d_at() instead? Or rather, make that store a
> store-release?

I prefer it this way too, but I suspected the majority would be
concerned with the performance implications, especially those
looping set_pte_at()s in mm/huge_memory.c.

If we have a consensus that smp_wmb() would be better off wrapped
together with set_p*d_at() in a macro, I'd be glad to make the change.

And it seems to me applying smp_store_release() would have to happen
in each arch, which might be just as fragile.