linux-kernel - Re: [v3][PATCH 2/2] mm: thp: give transparent hugepage code a separate copy

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Date:	Mon, 18 Nov 2013 10:32:47 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	dave.jiang@...el.com, akpm@...ux-foundation.org, dhillf@...il.com,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [v3][PATCH 2/2] mm: thp: give transparent hugepage code a
 separate copy_page

On Fri, Nov 15, 2013 at 02:55:53PM -0800, Dave Hansen wrote:
> 
> Changes from v2:
>  * 
> Changes from v1:
>  * removed explicit might_sleep() in favor of the one that we
>    get from the cond_resched();
> 
> --
> 
> 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)) {
> ...
> 
> Mel noticed that the migration code is really the only user of
> these functions.  This moves all the copy code over to migrate.c
> and makes copy_huge_page() work for THP by checking for it
> explicitly.
> 
> I believe the bug was introduced in b32967ff101:
> Author: Mel Gorman <mgorman@...e.de>
> Date:   Mon Nov 19 12:35:47 2012 +0000
> mm: numa: Add THP migration for the NUMA working set scanning fault case.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs
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