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

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Date:	Thu, 23 May 2013 15:33:18 +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 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);

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
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