Re: THP, rmap and page_referenced_one()

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 08, 2011 at 01:58:30PM +0100, Andrea Arcangeli wrote:
> When vmscan.c calls page_referenced, if an anon page was created before a
> process forked, rmap will search for it in both of the processes, even though
> one of them might have since broken COW. If the child process mlocks the vma
> where the COWed page belongs to, page_referenced() running on the page mapped
> by the parent would lead to *vm_flags getting VM_LOCKED set erroneously (leading
> to the references on the parent page being ignored and evicting the parent page
> too early).
> 
> *mapcount would also be decremented by page_referenced_one even if the page
> wasn't found by page_check_address.
> 
> This also let pmdp_clear_flush_young_notify() go ahead on a
> pmd_trans_splitting() pmd. We hold the page_table_lock so
> __split_huge_page_map() must wait the pmdp_clear_flush_young_notify() to
> complete before it can modify the pmd. The pmd is also still mapped in userland
> so the young bit may materialize through a tlb miss before split_huge_page_map
> runs. This will provide a more accurate page_referenced() behavior during
> split_huge_page().
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Reported-by: Michel Lespinasse <walken@google.com>
> Reviewed-by: Michel Lespinasse <walken@google.com>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>

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