Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

[Doug Doan <dougd@cray.com>]

On 06/03/2010 11:11 AM, Andrew Morton wrote:
> On Thu, 3 Jun 2010 10:36:00 -0700
> Doug Doan<dougd@cray.com>  wrote:
>
>>> Well, specifically it means that
>>> mmu_notifier_invalidate_range_start/end() implemetnations can no longer
>>> take page_table_lock or any lock which nests outside page_table_lock.
>>> That lessens flexibility.
>>>
>>> As the other mmu_notifier_invalidate_range_start/end() callsite in this
>>> function carefully nested those calls outside page_table_lock, perhaps
>>> that was thought to be a significant thing.
>>
>> Here's my rationale: for the normal page case, the invalidation call is done
>> inside a page_table_lock,
>
> It is?  Where does that happen?

handle_pte_fault() acquires the lock before calling do_wp_page():

         ptl = pte_lockptr(mm, pmd);
         spin_lock(ptl);
         if (unlikely(!pte_same(*pte, entry)))
                 goto unlock;
         if (flags & FAULT_FLAG_WRITE) {
                 if (!pte_write(entry))
                         return do_wp_page(mm, vma, address,
                                         pte, pmd, ptl, entry);
                 entry = pte_mkdirty(entry);
         }

do_wp_page() calls set_pte_at_notify(), which either calls 
mmu_notifier_change_pte() or mmu_notifier_invalidate_page().

>
>> so the same should also be done in the huge page case.
>> Does it really make sense to call invalidation on one hugepage and have another
>> call invalidate the same hugepage while the first call is still not finished?