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riscv/mm: Adjust PAGE_PROT_NONE to comply with THP semantics

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This is a preparation for enabling THP migration.
As the commit b65399f6111b("arm64/mm: Change THP helpers
to comply with generic MM semantics") mentioned, pmd_present()
and pmd_trans_huge() are expected to behave in the following
manner:
-------------------------------------------------------------------------
|	PMD states	|	pmd_present	|	pmd_trans_huge	|
-------------------------------------------------------------------------
|	Mapped		|	Yes		|	Yes		|
-------------------------------------------------------------------------
|	Splitting	|	Yes		|	Yes		|
-------------------------------------------------------------------------
|	Migration/Swap	|	No		|	No		|
-------------------------------------------------------------------------

At present the PROT_NONE bit reuses the READ bit could not comply with
above semantics with two problems:
1. When splitting a PMD THP, PMD is first invalidated with
pmdp_invalidate()->pmd_mkinvalid(), which clears the PRESENT bit
and PROT_NONE bit/READ bit, if the PMD is read-only, then the PAGE_LEAF
property is also cleared, which results in pmd_present() return false.
2. When migrating, the swap entry only clear the PRESENT bit
and PROT_NONE bit/READ bit, the W/X bit may be set, so _PAGE_LEAF may be
true which results in pmd_present() return true.

Solution:
Adjust PROT_NONE bit from READ to GLOBAL bit can satisfy the above rules:
1. GLOBAL bit has no other meanings, not like the R/W/X bit, which is
also relative with _PAGE_LEAF property.
2. GLOBAL bit is at bit 5, making swap entry start from bit 6, bit 0-5
are zero, which means the PRESENT, PROT_NONE, and PAGE_LEAF are
all false, then the pmd_present() and pmd_trans_huge() return false when
in migration/swap.

Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Nanyong Sun 2021-11-23 22:06:37 +08:00 committed by Palmer Dabbelt
parent 8ee304396e
commit fba88ede6a
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GPG Key ID: 2E1319F35FBB1889
2 changed files with 7 additions and 6 deletions
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2
arch/riscv/include/asm/pgtable-bits.h
View File

@ -31,7 +31,7 @@
* _PAGE_PROT_NONE is set on not-present pages (and ignored by the hardware) to
* distinguish them from swapped out pages
*/
#define _PAGE_PROT_NONE _PAGE_READ
#define _PAGE_PROT_NONE _PAGE_GLOBAL
#define _PAGE_PFN_SHIFT 10

11
arch/riscv/include/asm/pgtable.h
View File

@ -119,7 +119,7 @@
/* Page protection bits */
#define _PAGE_BASE (_PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_USER)
#define PAGE_NONE __pgprot(_PAGE_PROT_NONE)
#define PAGE_NONE __pgprot(_PAGE_PROT_NONE | _PAGE_READ)
#define PAGE_READ __pgprot(_PAGE_BASE | _PAGE_READ)
#define PAGE_WRITE __pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_WRITE)
#define PAGE_EXEC __pgprot(_PAGE_BASE | _PAGE_EXEC)
@ -628,11 +628,12 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
*
* Format of swap PTE:
* bit 0: _PAGE_PRESENT (zero)
* bit 1: _PAGE_PROT_NONE (zero)
* bits 2 to 6: swap type
* bits 7 to XLEN-1: swap offset
* bit 1 to 3: _PAGE_LEAF (zero)
* bit 5: _PAGE_PROT_NONE (zero)
* bits 6 to 10: swap type
* bits 10 to XLEN-1: swap offset
*/
#define __SWP_TYPE_SHIFT 2
#define __SWP_TYPE_SHIFT 6
#define __SWP_TYPE_BITS 5
#define __SWP_TYPE_MASK ((1UL << __SWP_TYPE_BITS) - 1)
#define __SWP_OFFSET_SHIFT (__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)