linux-kernel - Re: [PATCH 22/23] thp, mm: split huge page on mmap file page

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Date:	Fri,  9 Aug 2013 17:50:06 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Khalid Aziz <khalid.aziz@...cle.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, Dave Hansen <dave@...1.net>,
	Ning Qu <quning@...gle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 22/23] thp, mm: split huge page on mmap file page

Khalid Aziz wrote:
> On Sun, 2013-08-04 at 05:17 +0300, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > We are not ready to mmap file-backed tranparent huge pages. Let's split
> > them on fault attempt.
> > 
> > Later we'll implement mmap() properly and this code path be used for
> > fallback cases.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  mm/filemap.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index ed65af5..f7857ef 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1743,6 +1743,8 @@ retry_find:
> >  			goto no_cached_page;
> >  	}
> >  
> > +	if (PageTransCompound(page))
> > +		split_huge_page(compound_trans_head(page));
> 
> Since PageTransCompound(page) returns true for transparent huge pages as
> well as hugetlbfs pages, could this code split hugetlbfs pages on an
> mmap() on to hugetlbfs pages? hugetlbfs pages are not supposed to be
> split, right?

hugetlbfs page cannot (should not) be here. hugetlb uses different
vm_ops->fault handeler.

-- 
 Kirill A. Shutemov
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