Re: [PATCH 02/10] mm/hugetlb: Add PGD based implementation awareness

On 04/11/2016 10:55 AM, Anshuman Khandual wrote:

On 04/07/2016 02:34 PM, Balbir Singh wrote:

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On 07/04/16 15:37, Anshuman Khandual wrote:

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Currently the config ARCH_WANT_GENERAL_HUGETLB enabled functions like
'huge_pte_alloc' and 'huge_pte_offset' dont take into account HugeTLB
page implementation at the PGD level. This is also true for functions
like 'follow_page_mask' which is called from move_pages() system call.
This lack of PGD level huge page support prohibits some architectures
to use these generic HugeTLB functions.

From what I know of move_pages(), it will always call follow_page_mask()
with FOLL_GET (I could be wrong here) and the implementation below
returns NULL for follow_huge_pgd().

You are right. This patch makes ARCH_WANT_GENERAL_HUGETLB functions aware
of PGD implementation so that we can do all transactions on 16GB pages
using these function instead of the present arch overrides. But that also
requires follow_page_mask() changes for every other access to the page
than the migrate_pages() usage.

But yes, we dont support migrate_pages() on PGD based pages yet, hence
it just returns NULL in that case. May be the commit message needs to
reflect this.

The next commit actually changes follow_huge_pud|pgd() functions to
support FOLL_GET and PGD based huge page migration.