linux-kernel - Re: [PATCH v1 05/11] mm: thp: check pmd migration entry in common path

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Date:	Thu, 3 Mar 2016 13:50:58 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	linux-mm@...ck.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1 05/11] mm: thp: check pmd migration entry in common
 path

On Thu, Mar 03, 2016 at 04:41:52PM +0900, Naoya Horiguchi wrote:
> If one of callers of page migration starts to handle thp, memory management code
> start to see pmd migration entry, so we need to prepare for it before enabling.
> This patch changes various code point which checks the status of given pmds in
> order to prevent race between thp migration and the pmd-related works.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> ---
>  arch/x86/mm/gup.c  |  3 +++
>  fs/proc/task_mmu.c | 25 +++++++++++++--------
>  mm/gup.c           |  8 +++++++
>  mm/huge_memory.c   | 66 ++++++++++++++++++++++++++++++++++++++++++++++++------
>  mm/memcontrol.c    |  2 ++
>  mm/memory.c        |  5 +++++
>  6 files changed, 93 insertions(+), 16 deletions(-)
> 
> diff --git v4.5-rc5-mmotm-2016-02-24-16-18/arch/x86/mm/gup.c v4.5-rc5-mmotm-2016-02-24-16-18_patched/arch/x86/mm/gup.c
> index f8d0b5e..34c3d43 100644
> --- v4.5-rc5-mmotm-2016-02-24-16-18/arch/x86/mm/gup.c
> +++ v4.5-rc5-mmotm-2016-02-24-16-18_patched/arch/x86/mm/gup.c
> @@ -10,6 +10,7 @@
>  #include <linux/highmem.h>
>  #include <linux/swap.h>
>  #include <linux/memremap.h>
> +#include <linux/swapops.h>
>  
>  #include <asm/pgtable.h>
>  
> @@ -210,6 +211,8 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>  		if (pmd_none(pmd))
>  			return 0;
>  		if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
> +			if (unlikely(is_pmd_migration_entry(pmd)))
> +				return 0;

Hm. I've expected to see bunch of pmd_none() to pmd_present() conversions.
That's seems a right way guard the code. Otherwise we wound need even more
checks once PMD-level swap is implemented.

I think we need to check for migration entires only if we have something
to do with migration. In all other cases pmd_present() should be enough to
bail out.

-- 
 Kirill A. Shutemov