Re: [PATCH v2] mm, futex: Fix shared futex pgoff on shmem huge page

[Matthew Wilcox]

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@xxxxxxxxxx>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>