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

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Message-ID: <4C07F5EC.20600@cray.com>
Date:	Thu, 3 Jun 2010 11:35:24 -0700
From:	Doug Doan <dougd@...y.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"lee.schermerhorn@...com" <lee.schermerhorn@...com>,
	"rientjes@...gle.com" <rientjes@...gle.com>,
	"mel@....ul.ie" <mel@....ul.ie>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

On 06/03/2010 11:11 AM, Andrew Morton wrote:
> On Thu, 3 Jun 2010 10:36:00 -0700
> Doug Doan<dougd@...y.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?
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