From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx194.postini.com [74.125.245.194]) by kanga.kvack.org (Postfix) with SMTP id 610A76B0002 for ; Thu, 23 May 2013 08:31:11 -0400 (EDT) From: "Kirill A. Shutemov" In-Reply-To: References: <1368321816-17719-1-git-send-email-kirill.shutemov@linux.intel.com> <1368321816-17719-38-git-send-email-kirill.shutemov@linux.intel.com> <20130523120856.DD752E0090@blue.fi.intel.com> Subject: Re: [PATCHv4 37/39] thp: handle write-protect exception to file-backed huge pages Content-Transfer-Encoding: 7bit Message-Id: <20130523123318.A5285E0090@blue.fi.intel.com> Date: Thu, 23 May 2013 15:33:18 +0300 (EEST) Sender: owner-linux-mm@kvack.org List-ID: To: Hillf Danton Cc: "Kirill A. Shutemov" , Andrea Arcangeli , Andrew Morton , Al Viro , Hugh Dickins , Wu Fengguang , Jan Kara , Mel Gorman , linux-mm@kvack.org, Andi Kleen , Matthew Wilcox , "Kirill A. Shutemov" , Dave Hansen , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Hillf Danton wrote: > On Thu, May 23, 2013 at 8:08 PM, Kirill A. Shutemov > wrote: > > Hillf Danton wrote: > >> On Sun, May 12, 2013 at 9:23 AM, Kirill A. Shutemov > >> 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); It makes sense for hugetlb, since it RAM-backed only. Currently, the project supports only ramfs, but I hope we will bring storage-backed filesystems later. For them it would be much cheaper to copy the page then bring it back later from storage. And one more point: we must not ever reuse dirty pages, since it will lead to data lost. And ramfs pages are always dirty. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org