On Wed, 11 Dec 2013, Mel Gorman wrote:
> On Tue, Dec 10, 2013 at 11:42:15PM +0100, Thomas Gleixner wrote:
> > Now, if that map is RO, i.e. we took the fallback path then the THP
> > one will fail as it has write=1 unconditionally.
> >
> > if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1))
> >
>
> Not that it really matters but the naming and comments around that
> particular __get_user_pages_fast call are a little misleading. The ifdef
> CONFIG_TRANSPARENT_HUGEPAGE in futex.c is there because greater care has
> to be taken against THP splits, not because it is dealing exclusively with
> THP. The PageTail check applies to either hugetlbfs or THPs and similarly
> gup_huge_pmd handles both. The whole path should be very rare for THPs
> considering that THPs exist on MAP_PRIVATE anonymous mappings and how many
> shared futexes exist backed by such mappings? A RO mapping makes that seem
> even more strange because what thread is updating the value the caller is
> waiting on? It seems more like that was a shared futex on a hugetlbfs-backed
> mappings which might explain why the bug was undiscovered for so long.
This is the fshared path. The MAP_PRIVATE path does not do that dance
at all.
Thanks,
tglx
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