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

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Message-ID: <570B3FDB.90305@linux.vnet.ibm.com>
Date:	Mon, 11 Apr 2016 11:40:35 +0530
From:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:	Balbir Singh <bsingharora@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
CC:	hughd@...gle.com, dave.hansen@...el.com,
	aneesh.kumar@...ux.vnet.ibm.com, kirill@...temov.name,
	n-horiguchi@...jp.nec.com, mgorman@...hsingularity.net,
	akpm@...ux-foundation.org
Subject: 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:
>> > 
>> > 
>> > On 07/04/16 15:37, Anshuman Khandual wrote:
>>> >> 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.