Re: process 'stuck' at exit.

[Mel Gorman]

On Wed, Dec 11, 2013 at 05:38:55PM +0100, Thomas Gleixner wrote:
> 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.
> 

do_futex takes an op parameter and sets FLAGS_SHARED on that op parameter,
not whether the VMA backing that address was created with the MAP_PRIVATE
or MAP_SHARED flag. For example;

do_futex(addr, op) where !(ops & FUTEX_PRIVATE_FLAG) and cmd == FUTEX_WAIT
  -> futex_wait
    -> futex_wait_setup
      -> get_futex_key(fshared == true)

THP pages encountered in the fshared path should be rare because why create
a shared futex on a private mapping? That does not make it impossible
which is why splits are guarded against. If it was genuinely impossible
to be in this path for MAP_PRIVATE then there is no point worrying about
THP as it is currently supported at least. Overall, it still seems more
likely this was a hugetlbfs page.

-- 
Mel Gorman
SUSE Labs
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