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arm64: Ignore hardware dirty bit updates in ptep_set_wrprotect()

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ptep_set_wrprotect() is only called on CoW mappings which are private
(!VM_SHARED) with the pte either read-only (!PTE_WRITE && PTE_RDONLY) or
writable and software-dirty (PTE_WRITE && !PTE_RDONLY && PTE_DIRTY).
There is no race with the hardware update of the dirty state: clearing
of PTE_RDONLY when PTE_WRITE (a.k.a. PTE_DBM) is set. This patch removes
the code setting the software PTE_DIRTY bit in ptep_set_wrprotect() as
superfluous. A VM_WARN_ONCE is introduced in case the above logic is
wrong or the core mm code changes its use of ptep_set_wrprotect().

Reviewed-by: Will Deacon <will.deacon@arm.com>
Acked-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Catalin Marinas 2017-07-05 10:59:42 +01:00
parent 73e86cb03c
commit 64c26841b3
1 changed files with 13 additions and 8 deletions
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21
arch/arm64/include/asm/pgtable.h
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@ -634,23 +634,28 @@ static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
/*
* ptep_set_wrprotect - mark read-only while trasferring potential hardware
* dirty status (PTE_DBM && !PTE_RDONLY) to the software PTE_DIRTY bit.
* ptep_set_wrprotect - mark read-only while preserving the hardware update of
* the Access Flag.
*/
#define __HAVE_ARCH_PTEP_SET_WRPROTECT
static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long address, pte_t *ptep)
{
pte_t old_pte, pte;
/*
* ptep_set_wrprotect() is only called on CoW mappings which are
* private (!VM_SHARED) with the pte either read-only (!PTE_WRITE &&
* PTE_RDONLY) or writable and software-dirty (PTE_WRITE &&
* !PTE_RDONLY && PTE_DIRTY); see is_cow_mapping() and
* protection_map[]. There is no race with the hardware update of the
* dirty state: clearing of PTE_RDONLY when PTE_WRITE (a.k.a. PTE_DBM)
* is set.
*/
VM_WARN_ONCE(pte_write(*ptep) && !pte_dirty(*ptep),
"%s: potential race with hardware DBM", __func__);
pte = READ_ONCE(*ptep);
do {
old_pte = pte;
/*
* If hardware-dirty (PTE_WRITE/DBM bit set and PTE_RDONLY
* clear), set the PTE_DIRTY bit.
*/
if (pte_hw_dirty(pte))
pte = pte_mkdirty(pte);
pte = pte_wrprotect(pte);
pte_val(pte) = cmpxchg_relaxed(&pte_val(*ptep),
pte_val(old_pte), pte_val(pte));