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Commit 64c26841 authored by Catalin Marinas's avatar Catalin Marinas
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arm64: Ignore hardware dirty bit updates in ptep_set_wrprotect() 



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: default avatarWill Deacon <will.deacon@arm.com>
Acked-by: default avatarSteve Capper <steve.capper@arm.com>
Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 73e86cb0

arch/arm64/include/asm/pgtable.h

+13 −8
Original line number Diff line number Diff line
@@ -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));