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arm64/mm: Change THP helpers to comply with generic MM semantics

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pmd_present() and pmd_trans_huge() are expected to behave in the following
manner during various phases of a given PMD. It is derived from a previous
detailed discussion on this topic [1] and present THP documentation [2].

pmd_present(pmd):

- Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
- Returns false if pmd refers to a migration or swap entry

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_mkinvalid -> Clears PMD_SECT_VALID

Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false
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 with
above mentioned semantics, pmd_trans_huge() should also check pmd_present()
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. Hence
it will not be there for pmd_present() check after pmdp_invalidate().

A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD
entry during invalidation which can help pmd_present() return true and in
recognizing the fact that it still points to memory.

This bit is transient. During the split process it will be overridden by a
page table page representing normal pages in place of erstwhile huge page.
Other pmdp_invalidate() callers always write a fresh PMD value on the entry
overriding this transient PMD_PRESENT_INVALID bit, which makes it safe.

[1]: https://lkml.org/lkml/2018/10/17/231
[2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/1599627183-14453-2-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Anshuman Khandual
2020-09-09 10:23:02 +05:30
committed by Will Deacon
parent c048ddf86c
commit b65399f611
2 changed files with 38 additions and 3 deletions
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7
arch/arm64/include/asm/pgtable-prot.h
View File

@@ -19,6 +19,13 @@
#define PTE_DEVMAP (_AT(pteval_t, 1) << 57)
#define PTE_PROT_NONE (_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */
/*
* This bit indicates that the entry is present i.e. pmd_page()
* still points to a valid huge page in memory even if the pmd
* has been invalidated.
*/
#define PMD_PRESENT_INVALID (_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
#ifndef __ASSEMBLY__
#include <asm/cpufeature.h>