linux-kernel - Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

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Date:   Tue, 09 Oct 2018 09:58:36 -0400
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Anshuman Khandual" <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org,
        mhocko@...e.com, will.deacon@....com,
        "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and
 PMD migration entry

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@....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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