linux-kernel - Re: [PATCHv4 37/39] thp: handle write-protect exception to file-backed huge pages

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Message-Id: <20130523120856.DD752E0090@blue.fi.intel.com>
Date:	Thu, 23 May 2013 15:08:56 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Hillf Danton <dhillf@...il.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 <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Dave Hansen <dave@...1.net>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 37/39] thp: handle write-protect exception to
 file-backed huge pages

Hillf Danton wrote:
> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> > @@ -1120,7 +1119,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >
> >         page = pmd_page(orig_pmd);
> >         VM_BUG_ON(!PageCompound(page) || !PageHead(page));
> > -       if (page_mapcount(page) == 1) {
> > +       if (PageAnon(page) && page_mapcount(page) == 1) {
> 
> Could we avoid copying huge page if
> no-one else is using it, no matter anon?

No. The page is still in page cache and can be later accessed later.
We could isolate the page from page cache, but I'm not sure whether it's
good idea.

do_wp_page() does exectly the same for small pages, so let's keep it
consistent.

-- 
 Kirill A. Shutemov
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