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Date:   Sun, 13 Jun 2021 11:27:26 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.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>,
        Thomas Gleixner <tglx@linutronix.de>,
        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
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On Sat, Jun 12, 2021 at 08:16:58PM -0700, 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>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>