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

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Date:	Wed, 2 Jun 2010 16:33:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Doug Doan <dougd@...y.com>
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 <andrea@...hat.com>
Subject: Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

On Wed, 2 Jun 2010 16:13:42 -0700
Doug Doan <dougd@...y.com> wrote:

> On 06/01/2010 11:16 PM, Andrew Morton wrote:
> > On Thu, 27 May 2010 13:43:00 -0700 Doug Doan<dougd@...y.com>  wrote:
> >
> >>
> >> When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
> >> through handle_pte_fault for normal pages, or through hugetlb_fault for huge pages.
> >>
> >> In the normal page case, we eventually get to do_wp_page and call mmu notifiers
> >> via ptep_clear_flush_notify. There is no callout to the mmmu notifiers in the
> >> huge page case. This patch fixes that.
> >>
> >> Signed-off-by: Doug Doan<dougd@...y.com>
> >> ---
> >>
> >> [patch  text/plain (802B)]
> >> --- mm/hugetlb.c.orig	2010-05-27 13:07:58.569546314 -0700
> >> +++ mm/hugetlb.c	2010-05-26 14:41:06.449296524 -0700
> >
> > (In patch -p1 form, please.  So a/mm/hugetlb.c)
> >
> >> @@ -2345,11 +2345,17 @@ retry_avoidcopy:
> >>   	ptep = huge_pte_offset(mm, address&  huge_page_mask(h));
> >>   	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> >>   		/* Break COW */
> >> +		mmu_notifier_invalidate_range_start(mm,
> >> +			address&  huge_page_mask(h),
> >> +			(address&  huge_page_mask(h)) + huge_page_size(h));
> >>   		huge_ptep_clear_flush(vma, address, ptep);
> >>   		set_huge_pte_at(mm, address, ptep,
> >>   				make_huge_pte(vma, new_page, 1));
> >>   		/* Make the old page be freed below */
> >>   		new_page = old_page;
> >> +		mmu_notifier_invalidate_range_end(mm,
> >> +			address&  huge_page_mask(h),
> >> +			(address&  huge_page_mask(h)) + huge_page_size(h));
> >>   	}
> >>   	page_cache_release(new_page);
> >>   	page_cache_release(old_page);
> >
> > This causes mmu_notifier_invalidate_range_start() to be called under
> > page_table_lock.  The immediately preceding code seems to take some
> > care to avoid doing that.  I took a quick look at other callsites and
> > cannot immediately see other cases where
> > mmu_notifier_invalidate_range_start/end() are called under that lock.
> >
> > This may not introduce bugs with current notifier implementations (I
> > didn't check), but it does lessen flexibility?
> 
> In the normal page case, handle_pte_fault calls do_wp_page inside a spinlock on 
> ptl = pte_lockptr(mm, pmd), which uses mm->page_table_lock if USE_SPLIT_PTLOCKS 
> is not defined.
> 
> I don't understand what you mean by lessen flexibilty.

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.
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