On Wed 22-11-17 09:43:46, Zi Yan wrote:

Michal Hocko wrote:

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On Wed 22-11-17 09:54:16, Michal Hocko wrote:

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On Mon 20-11-17 21:18:55, Zi Yan wrote:

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diff --git a/include/linux/migrate.h b/include/linux/migrate.h index 895ec0c4942e..a2246cf670ba 100644 --- a/include/linux/migrate.h +++ b/include/linux/migrate.h @@ -54,7 +54,7 @@ static inline struct page *new_page_nodemask(struct page *page, new_page = __alloc_pages_nodemask(gfp_mask, order, preferred_nid, nodemask);

• if (new_page && PageTransHuge(page))

• if (new_page && PageTransHuge(new_page)) prep_transhuge_page(new_page);

I would keep the two checks consistent. But that leads to a more interesting question. new_page_nodemask does

if (thp_migration_supported() && PageTransHuge(page)) { order = HPAGE_PMD_ORDER; gfp_mask |= GFP_TRANSHUGE; }

And one more question/note. Why do we need thp_migration_supported in the first place? 9c670ea37947 ("mm: thp: introduce CONFIG_ARCH_ENABLE_THP_MIGRATION") says : Introduce CONFIG_ARCH_ENABLE_THP_MIGRATION to limit thp migration : functionality to x86_64, which should be safer at the first step.

but why is unsafe to enable the feature on other arches which support THP? Is there any plan to do the next step and remove this config option?

Because different architectures have their own way of specifying a swap entry. This means, to support THP migration, each architecture needs to add its own __pmd_to_swp_entry() and __swp_entry_to_pmd(), which are used for arch-independent pmd_to_swp_entry() and swp_entry_to_pmd().

I understand that part. But this smells like a matter of coding, no? I was suprised to see the note about safety which didn't make much sense to me.