linux-kernel - Re: [PATCH 9/8] hugetlb: add pmd_huge_support() to migrate only pmd-based hugepage

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Message-ID: <87eha7oa4l.fsf@linux.vnet.ibm.com>
Date:	Tue, 06 Aug 2013 07:26:10 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, Michal Hocko <mhocko@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH 9/8] hugetlb: add pmd_huge_support() to migrate only pmd-based hugepage

Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:

> This patch is motivated by the discussion with Aneesh about "extend
> hugepage migration" patchset.
>   http://thread.gmane.org/gmane.linux.kernel.mm/103933/focus=104391
> I'll append this to the patchset in the next post, but before that
> I want this patch to be reviewed (I don't want to repeat posting the
> whole set for just minor changes.)
>
> Any comments?
>
> Thanks,
> Naoya Horiguchi
> ---
> From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Date: Mon, 5 Aug 2013 13:33:02 -0400
> Subject: [PATCH] hugetlb: add pmd_huge_support() to migrate only pmd-based
>  hugepage
>
> Currently hugepage migration works well only for pmd-based hugepages,
> because core routines of hugepage migration use pmd specific internal
> functions like huge_pte_offset(). So we should not enable the migration
> of other levels of hugepages until we are ready for it.

I guess huge_pte_offset may not be the right reason because archs do
implement huge_pte_offsets even if they are not pmd-based hugepages

pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
{
	/* Only called for hugetlbfs pages, hence can ignore THP */
	return find_linux_pte_or_hugepte(mm->pgd, addr, NULL);
}

>
> Some users of hugepage migration (mbind, move_pages, and migrate_pages)
> do page table walk and check pud/pmd_huge() there, so they are safe.
> But the other users (softoffline and memory hotremove) don't do this,
> so they can try to migrate unexpected types of hugepages.
>
> To prevent this, we introduce an architecture dependent check of whether
> hugepage are implemented on a pmd basis or not. It returns 0 if pmd_huge()
> returns always 0, and 1 otherwise.
>

so why not #define pmd_huge_support pmd_huge or use pmd_huge directly ?

> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> ---
>  arch/arm/mm/hugetlbpage.c     |  5 +++++
>  arch/arm64/mm/hugetlbpage.c   |  5 +++++
>  arch/ia64/mm/hugetlbpage.c    |  5 +++++
>  arch/metag/mm/hugetlbpage.c   |  5 +++++
>  arch/mips/mm/hugetlbpage.c    |  5 +++++
>  arch/powerpc/mm/hugetlbpage.c | 10 ++++++++++
>  arch/s390/mm/hugetlbpage.c    |  5 +++++
>  arch/sh/mm/hugetlbpage.c      |  5 +++++
>  arch/sparc/mm/hugetlbpage.c   |  5 +++++
>  arch/tile/mm/hugetlbpage.c    |  5 +++++
>  arch/x86/mm/hugetlbpage.c     |  8 ++++++++
>  include/linux/hugetlb.h       |  2 ++
>  mm/migrate.c                  | 11 +++++++++++
>  13 files changed, 76 insertions(+)
>
> diff --git a/arch/arm/mm/hugetlbpage.c b/arch/arm/mm/hugetlbpage.c
> index 3d1e4a2..3f3b6a7 100644
> --- a/arch/arm/mm/hugetlbpage.c
> +++ b/arch/arm/mm/hugetlbpage.c
> @@ -99,3 +99,8 @@ int pmd_huge(pmd_t pmd)
>  {
>  	return pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT);
>  }
> +
> +int pmd_huge_support(void)
> +{
> +	return 1;
> +}

-aneesh

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