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

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Message-ID: <20131028221126.GA29431@shutemov.name>
Date:	Tue, 29 Oct 2013 00:11:26 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Dave Hansen <dave@...1.net>
Cc:	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 separate
 copy_page

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.

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

-- 
 Kirill A. Shutemov
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