linux-kernel - Re: [PATCH 2/2] mm: thp: give transparent hugepage code a separatecopy

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Date:	Wed, 30 Oct 2013 11:31:38 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Dave Hansen <dave@...1.net>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, dave.jiang@...el.com,
	Mel Gorman <mgorman@...e.de>, akpm@...ux-foundation.org,
	dhillf@...il.com
Subject: Re: [PATCH 2/2] mm: thp: give transparent hugepage code a
 separatecopy_page

On Tue, Oct 29, 2013 at 12:11:26AM +0200, Kirill A. Shutemov wrote:
> On Mon, Oct 28, 2013 at 03:16:20PM -0700, Dave Hansen wrote:
> > 
> > From: Dave Hansen <dave.hansen@...ux.intel.com>
> > 
> > Right now, the migration code in migrate_page_copy() uses 
> > copy_huge_page() for hugetlbfs and thp pages:
> > 
> >        if (PageHuge(page) || PageTransHuge(page))
> >                 copy_huge_page(newpage, page);
> > 
> > So, yay for code reuse.  But:
> > 
> > void copy_huge_page(struct page *dst, struct page *src)
> > {
> >         struct hstate *h = page_hstate(src);
> > 
> > and a non-hugetlbfs page has no page_hstate().  This
> > works 99% of the time because page_hstate() determines
> > the hstate from the page order alone.  Since the page
> > order of a THP page matches the default hugetlbfs page
> > order, it works.
> > 
> > But, if you change the default huge page size on the
> > boot command-line (say default_hugepagesz=1G), then
> > we might not even *have* a 2MB hstate so page_hstate()
> > returns null and copy_huge_page() oopses pretty fast
> > since copy_huge_page() dereferences the hstate:
> > 
> > void copy_huge_page(struct page *dst, struct page *src)
> > {
> >         struct hstate *h = page_hstate(src);
> >         if (unlikely(pages_per_huge_page(h) > MAX_ORDER_NR_PAGES)) {
> > ...
> > 
> > This patch creates a copy_high_order_page() which can
> > be used on THP pages.
> 
> We already have copy_user_huge_page() and copy_user_gigantic_page() in
> generic code (mm/memory.c). I think copy_gigantic_page() and
> copy_huge_page() should be moved there too.

I agree this.

> BTW, I think pages_per_huge_page in copy_user_huge_page() is redunand:
> compound_order(page) should be enough, right?

I guess that thp code is very strict on performance, so developers chose
to pass it as an argument instead of calculating compound_order in each call.
I think the performance gain is small (maybe invisible),
but it's not a bad idea to me.

Thanks,
Naoya
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