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Date: Thu, 23 May 2013 20:12:07 +0800
Message-ID: <CAJd=RBAcsAk5qN1dy2OhHYqB3fuwd0RfjPK_4=E1G_k=CUwycw@mail.gmail.com>
Subject: Re: [PATCHv4 37/39] thp: handle write-protect exception to
 file-backed huge pages
From: Hillf Danton <dhillf@gmail.com>
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Sender: owner-linux-mm@kvack.org
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Al Viro <viro@zeniv.linux.org.uk>, Hugh Dickins <hughd@google.com>, Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, Andi Kleen <ak@linux.intel.com>, Matthew Wilcox <matthew.r.wilcox@intel.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, Dave Hansen <dave@sr71.net>, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org

On Thu, May 23, 2013 at 8:08 PM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
> Hillf Danton wrote:
>> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov
>> <kirill.shutemov@linux.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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