linux-kernel - Re: [PATCH 01/10] migrate: add migrate_entry_wait

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Date:	Mon, 08 Apr 2013 16:00:12 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, Michal Hocko <mhocko@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/10] migrate: add migrate_entry_wait_huge()

On Fri, Apr 05, 2013 at 04:33:32PM -0400, KOSAKI Motohiro wrote:
> > diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
> > index 0a0be33..98a478e 100644
> > --- v3.9-rc3.orig/mm/hugetlb.c
> > +++ v3.9-rc3/mm/hugetlb.c
> > @@ -2819,7 +2819,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> >  	if (ptep) {
> >  		entry = huge_ptep_get(ptep);
> >  		if (unlikely(is_hugetlb_entry_migration(entry))) {
> > -			migration_entry_wait(mm, (pmd_t *)ptep, address);
> > +			migration_entry_wait_huge(mm, (pmd_t *)ptep, address);
> 
> Hm.
> 
> How do you test this? From x86 point of view, this patch seems unnecessary because
> hugetlb_fault call "address &= hugetlb_mask()" at first and then migration_entry_wait()
> could grab right pte lock. And from !x86 point of view, this funciton still doesn't work
> because huge page != pmd on some arch.

I kicked hugepage migration for address range where I repeat to access
in a loop, and checked what happened (whether soft lockup happens or not.)
But I don't fully understand what the problem is, and I might wrongly define
the problem. So give me time to clarify it.

And I fully agree that this function should be arch dependent.

> I might be missing though.
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