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

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Message-ID: <CAJd=RBAcsAk5qN1dy2OhHYqB3fuwd0RfjPK_4=E1G_k=CUwycw@mail.gmail.com>
Date:	Thu, 23 May 2013 20:12:07 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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

On Thu, May 23, 2013 at 8:08 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
> 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.
>
Hugetlb tries to avoid copying pahe.

	/* If no-one else is actually using this page, avoid the copy
	 * and just make the page writable */
	avoidcopy = (page_mapcount(old_page) == 1);
--
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