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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zhang Yi <wetpzy@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Neel Natu <neelnatu@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm, futex: Fix shared futex pgoff on shmem huge page
In-Reply-To: <b17d946b-d09-326e-b42a-52884c36df32@google.com>
References: <45e8fd67-51fd-7828-fe43-d261d6c33727@google.com> <YMTdtRZG+7q8OtkK@casper.infradead.org> <b17d946b-d09-326e-b42a-52884c36df32@google.com>
Date:   Tue, 15 Jun 2021 16:53:27 +0200
Message-ID: <87mtrrgowo.ffs@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Sat, Jun 12 2021 at 20:16, Hugh Dickins wrote:
> If more than one futex is placed on a shmem huge page, it can happen that
> waking the second wakes the first instead, and leaves the second waiting:
> the key's shared.pgoff is wrong.
>
> When 3.11 commit 13d60f4b6ab5 ("futex: Take hugepages into account when
> generating futex_key"), the only shared huge pages came from hugetlbfs,
> and the code added to deal with its exceptional page->index was put into
> hugetlb source.  Then that was missed when 4.8 added shmem huge pages.
>
> page_to_pgoff() is what others use for this nowadays: except that, as
> currently written, it gives the right answer on hugetlbfs head, but
> nonsense on hugetlbfs tails.  Fix that by calling hugetlbfs-specific
> hugetlb_basepage_index() on PageHuge tails as well as on head.
>
> Yes, it's unconventional to declare hugetlb_basepage_index() there in
> pagemap.h, rather than in hugetlb.h; but I do not expect anything but
> page_to_pgoff() ever to need it.
>
> Fixes: 800d8c63b2e9 ("shmem: add huge pages support")
> Reported-by: Neel Natu <neelnatu@google.com>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>

Assuming this goes through mm:

Acked-by: Thomas Gleixner <tglx@linutronix.de>