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From:   Zi Yan <ziy@nvidia.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
CC:     <linux-mm@kvack.org>, Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 1/2] arm64/mm: Change THP helpers to comply with generic MM
 semantics
Date:   Thu, 27 Jun 2019 11:31:31 -0400
Message-ID: <7F685152-7C6C-4E99-99DF-03DDD03D6094@nvidia.com>
In-Reply-To: <1561639696-16361-2-git-send-email-anshuman.khandual@arm.com>
References: <1561639696-16361-1-git-send-email-anshuman.khandual@arm.com>
 <1561639696-16361-2-git-send-email-anshuman.khandual@arm.com>
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On 27 Jun 2019, at 8:48, Anshuman Khandual wrote:

> pmd_present() and pmd_trans_huge() are expected to behave in the follow=
ing
> manner during various phases of a given PMD. It is derived from a previ=
ous
> detailed discussion on this topic [1] and present THP documentation [2]=
=2E
>
> pmd_present(pmd):
>
> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(=
pmd)
>
> pmd_trans_huge(pmd):
>
> - Returns true if pmd refers to system RAM and is a trans huge mapping
>
> -----------------------------------------------------------------------=
--
> |	PMD states	|	pmd_present	|	pmd_trans_huge	|
> -----------------------------------------------------------------------=
--
> |	Mapped		|	Yes		|	Yes		|
> -----------------------------------------------------------------------=
--
> |	Splitting	|	Yes		|	Yes		|
> -----------------------------------------------------------------------=
--
> |	Migration/Swap	|	No		|	No		|
> -----------------------------------------------------------------------=
--
>
> The problem:
>
> PMD is first invalidated with pmdp_invalidate() before it's splitting. =
This
> invalidation clears PMD_SECT_VALID as below.
>
> PMD Split -> pmdp_invalidate() -> pmd_mknotpresent -> Clears PMD_SECT_V=
ALID
>
> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return fa=
lse
> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to=
 re-
> affirm pmd_present() as true during the THP split process. To comply wi=
th
> above mentioned semantics, pmd_trans_huge() should also check pmd_prese=
nt()
> first before testing presence of an actual transparent huge mapping.
>
> The solution:
>
> Ideally PMD_TYPE_SECT should have been used here instead. But it shares=
 the
> bit position with PMD_SECT_VALID which is used for THP invalidation. He=
nce
> it will not be there for pmd_present() check after pmdp_invalidate().
>
> PTE_SPECIAL never gets used for PMD mapping i.e there is no pmd_special=
().
> Hence this bit can be set on the PMD entry during invalidation which ca=
n
> help in making pmd_present() return true and in recognizing the fact th=
at
> it still points to memory.
>
> This bit is transient. During the split is process it will be overridde=
n
> by a page table page representing the normal pages in place of erstwhil=
e
> huge page. Other pmdp_invalidate() callers always write a fresh PMD val=
ue
> on the entry overriding this transient PTE_SPECIAL making it safe. In t=
he
> past former pmd_[mk]splitting() functions used PTE_SPECIAL.
>
> [1]: https://lkml.org/lkml/2018/10/17/231

Just want to point out that lkml.org link might not be stable.
This one would be better: https://lore.kernel.org/linux-mm/20181017020930=
=2EGN30832@redhat.com/


=E2=80=94
Best Regards,
Yan Zi

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