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From: "Zi Yan" <zi.yan@cs.rutgers.edu>
To: "Anshuman Khandual" <anshuman.khandual@arm.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kirill.shutemov@linux.intel.com, akpm@linux-foundation.org,
	mhocko@suse.com, will.deacon@arm.com,
	"Naoya Horiguchi" <n-horiguchi@ah.jp.nec.com>
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry
Date: Tue, 09 Oct 2018 09:58:36 -0400	[thread overview]
Message-ID: <7E8E6B14-D5C4-4A30-840D-A7AB046517FB@cs.rutgers.edu> (raw)
In-Reply-To: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com>

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cc: Naoya Horiguchi (who proposed to use !_PAGE_PRESENT && !_PAGE_PSE for x86
PMD migration entry check)

On 8 Oct 2018, at 23:58, Anshuman Khandual wrote:

> A normal mapped THP page at PMD level should be correctly differentiated
> from a PMD migration entry while walking the page table. A mapped THP would
> additionally check positive for pmd_present() along with pmd_trans_huge()
> as compared to a PMD migration entry. This just adds a new conditional test
> differentiating the two while walking the page table.
>
> Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually
> exclusive which makes the current conditional block work for both mapped
> and migration entries. This is not same with arm64 where pmd_trans_huge()

!pmd_present() && pmd_trans_huge() is used to represent THPs under splitting,
since _PAGE_PRESENT is cleared during THP splitting but _PAGE_PSE is not.
See the comment in pmd_present() for x86, in arch/x86/include/asm/pgtable.h

> returns positive for both mapped and migration entries. Could some one
> please explain why pmd_trans_huge() has to return false for migration
> entries which just install swap bits and its still a PMD ? Nonetheless
> pmd_present() seems to be a better check to distinguish between mapped
> and (non-mapped non-present) migration entries without any ambiguity.

If arm64 does it differently, I just wonder how THP splitting is handled
in arm64.


--
Best Regards
Yan Zi

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  parent reply	other threads:[~2018-10-09 13:58 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-09  3:58 [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry Anshuman Khandual
2018-10-09 13:04 ` Kirill A. Shutemov
2018-10-09 13:18   ` Will Deacon
2018-10-12  8:02     ` Anshuman Khandual
2018-10-15  8:32       ` Aneesh Kumar K.V
2018-10-16 13:16         ` Anshuman Khandual
2018-10-09 13:42   ` Anshuman Khandual
2018-10-09 13:58 ` Zi Yan [this message]
2018-10-10  4:05   ` Anshuman Khandual
2018-10-10 12:43     ` Zi Yan
2018-10-12  8:00       ` Anshuman Khandual
2018-10-15  0:53         ` Zi Yan
2018-10-15  4:06           ` Anshuman Khandual
2018-10-16 14:31             ` Zi Yan
2018-10-18  2:17               ` Naoya Horiguchi
2018-11-02  5:22                 ` Anshuman Khandual
2018-10-25  8:10               ` Anshuman Khandual
2018-10-25 18:45                 ` Zi Yan
2018-10-26  1:39                   ` Anshuman Khandual
2018-10-17  2:09           ` Andrea Arcangeli
2018-10-22 14:00             ` Zi Yan
2018-11-02  6:15             ` Anshuman Khandual
2018-11-06  0:35               ` Will Deacon
2018-11-06  9:51                 ` Anshuman Khandual

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