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SubjectRe: [PATCH] mm, hugetlb: fix "treat_as_movable" condition in htlb_alloc_mask
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Adding Anshuman

On 09/29/2017 01:43 PM, Alexandru Moise wrote:
> On Fri, Sep 29, 2017 at 05:13:39PM +0200, Alexandru Moise wrote:
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 424b0ef08a60..ab28de0122af 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -926,7 +926,7 @@ static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask,
>> /* Movability of hugepages depends on migration support. */
>> static inline gfp_t htlb_alloc_mask(struct hstate *h)
>> {
>> - if (hugepages_treat_as_movable || hugepage_migration_supported(h))
>> + if (hugepages_treat_as_movable && hugepage_migration_supported(h))
>> return GFP_HIGHUSER_MOVABLE;
>> else
>> return GFP_HIGHUSER;
>> --
>> 2.14.2
>>
>
> I seem to have terribly misunderstood the semantics of this flag wrt hugepages,
> please ignore this for now.

That is Okay, it made me look at this code more closely.

static inline bool hugepage_migration_supported(struct hstate *h)
{
#ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
if ((huge_page_shift(h) == PMD_SHIFT) ||
(huge_page_shift(h) == PGDIR_SHIFT))
return true;
else
return false;
#else
return false;
#endif
}

So, hugepage_migration_supported() can only return true if
ARCH_ENABLE_HUGEPAGE_MIGRATION is defined. Commit c177c81e09e5
restricts hugepage_migration_support to x86_64. So,
ARCH_ENABLE_HUGEPAGE_MIGRATION is only defined for x86_64.

Commit 94310cbcaa3c added the ability to migrate gigantic hugetlb pages
at the PGD level. This added the check for PGD level pages to
hugepage_migration_supported(), which is only there if
ARCH_ENABLE_HUGEPAGE_MIGRATION is defined. IIUC, this functionality
was added for powerpc. Yet, powerpc does not define
ARCH_ENABLE_HUGEPAGE_MIGRATION (unless I am missing something).

--
Mike Kravetz

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