linux-kernel - Re: [PATCH 5/9] migrate: enable migrate

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Date:	Wed, 20 Mar 2013 02:12:54 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Michal Hocko <mhocko@...e.cz>
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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] migrate: enable migrate_pages() to migrate hugepage

On Tue, Mar 19, 2013 at 08:11:13AM +0100, Michal Hocko wrote:
> On Mon 18-03-13 20:07:16, Naoya Horiguchi wrote:
> > On Mon, Mar 18, 2013 at 04:40:57PM +0100, Michal Hocko wrote:
> > > On Thu 21-02-13 14:41:44, Naoya Horiguchi wrote:
...
> > > > @@ -536,6 +557,11 @@ static inline int check_pmd_range(struct vm_area_struct *vma, pud_t *pud,
> > > >  	pmd = pmd_offset(pud, addr);
> > > >  	do {
> > > >  		next = pmd_addr_end(addr, end);
> > > > +		if (pmd_huge(*pmd) && is_vm_hugetlb_page(vma)) {
> > > 
> > > Why an explicit check for is_vm_hugetlb_page here? Isn't pmd_huge()
> > > sufficient?
> > 
> > I think we need both check here because if we use only pmd_huge(),
> > pmd for thp goes into this branch wrongly. 
> 
> Bahh. You are right. I thought that pmd_huge is hugetlb thingy but it
> obviously checks only _PAGE_PSE same as pmd_large() which is really
> unfortunate and confusing. Can we make it hugetlb specific?

I agree that we had better fix this confusion.

What pmd_huge() (or pmd_large() in some architectures) does is just
checking whether a given pmd is pointing to huge/large page or not.
It does not say which type of hugepage it is.
So it shouldn't be used to decide whether the hugepage are hugetlbfs or not.
I think it would be better to introduce pmd_hugetlb() which has pmd and vma
as arguments and returns true only for hugetlbfs pmd.
Checking pmd_hugetlb() should come before checking pmd_trans_huge() because
pmd_trans_huge() implicitly assumes that the vma which covers the virtual
address of a given pmd is not hugetlbfs vma.

I'm interested in this cleanup, so will work on it after this patchset.

Thanks,
Naoya
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