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@xxxxxxxxxxxxxxx>
>
> 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@xxxxxxx>
> 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@xxxxxxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
--
Mel Gorman
SUSE Labs
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