[PATCH 5/6] arm64: Ignore hardware dirty bit updates in ptep_set_wrprotect()
From: catalin.marinas@arm.com (Catalin Marinas)
Date: 2017-07-25 13:53:07
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
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(). Cc: Will Deacon <redacted> Acked-by: Steve Capper <redacted> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> --- arch/arm64/include/asm/pgtable.h | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index a14e2120811c..3fefcc0182c7 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h@@ -632,23 +632,28 @@ 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__); do { - pte = old_pte = READ_ONCE(*ptep); - /* - * 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); + old_pte = READ_ONCE(*ptep); + pte = pte_wrprotect(old_pte); } while (cmpxchg_relaxed(&pte_val(*ptep), pte_val(old_pte), pte_val(pte)) != pte_val(old_pte)); }