Re: [PATCH v2] mm: migrate: fix missing update page_private to hugetlb_page_subpool

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/21/21 7:57 AM, Muchun Song wrote:
> Since commit d6995da31122 ("hugetlb: use page.private for hugetlb specific
> page flags") converts page.private for hugetlb specific page flags. We
> should use hugetlb_page_subpool() to get the subpool pointer instead of
> page_private().
> 
> This 'could' prevent the migration of hugetlb pages. page_private(hpage)
> is now used for hugetlb page specific flags.  At migration time, the
> only flag which could be set is HPageVmemmapOptimized.  This flag will
> only be set if the new vmemmap reduction feature is enabled.  In
> addition, !page_mapping() implies an anonymous mapping.  So, this will
> prevent migration of hugetb pages in anonymous mappings if the vmemmap
> reduction feature is enabled.
> 
> In addition, that if statement checked for the rare race condition of a
> page being migrated while in the process of being freed.  Since that
> check is now wrong, we could leak hugetlb subpool usage counts.
> 
> The commit forgot to update it in the page migration routine. So fix it.
> 
> Fixes: d6995da31122 ("hugetlb: use page.private for hugetlb specific page flags")
> Reported-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Tested on all page size configs and HugeTLB migrations work as expected.

Tested-by: Anshuman Khandual <anshuman.khandual@arm.com> #arm64