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

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Date:   Fri, 27 Sep 2019 14:33:00 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Yu Zhao <yuzhao@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "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 Thu 26-09-19 20:26:46, John Hubbard wrote:
> On 9/26/19 3:20 AM, Kirill A. Shutemov wrote:
> > BTW, have you looked at other levels of page table hierarchy. Do we have
> > the same issue for PMD/PUD/... pages?
> > 
> 
> Along the lines of "what other memory barriers might be missing for
> get_user_pages_fast(), I'm also concerned that the synchronization between
> get_user_pages_fast() and freeing the page tables might be technically broken,
> due to missing memory barriers on the get_user_pages_fast() side. Details:
> 
> gup_fast() disables interrupts, but I think it also needs some sort of
> memory barrier(s), in order to prevent reads of the page table (gup_pgd_range,
> etc) from speculatively happening before the interrupts are disabled. 

Could you be more specific about the race scenario please? I thought
that the unmap path will be serialized by the pte lock.
-- 
Michal Hocko
SUSE Labs