[RFC PATCH 14/14] mm: conditional check of pfn in pte_flush_type

Nadav Amit
2022-07-18T19:37:57Z

From: Nadav Amit <namit@vmware.com>

Checking whether PFNs in two PTEs are the same takes surprisingly large
number of instructions. Yet in fact, in most cases the caller to
pte_flush_type() already knows if the PFN was changed. For instance,
mprotect() does not change the PFN, but only modifies the protection
flags.

Add argument to pte_flush_type() to indicate whether the PFN should be
checked. Keep checking it in mm-debug to see if some caller was wrong to
assume the PFN is the same.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
arch/x86/include/asm/tlbflush.h | 14 ++++++++++----
include/asm-generic/tlb.h | 6 ++++--
mm/huge_memory.c | 2 +-
mm/mprotect.c | 2 +-
mm/rmap.c | 2 +-
5 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 58c95e36b098..50349861fdc9 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -340,14 +340,17 @@ static inline enum pte_flush_type pte_flags_flush_type(unsigned long oldflags,
* whether a strict or relaxed TLB flush is need. It should only be used on
* userspace PTEs.
*/
-static inline enum pte_flush_type pte_flush_type(pte_t oldpte, pte_t newpte)
+static inline enum pte_flush_type pte_flush_type(pte_t oldpte, pte_t newpte,
+ bool check_pfn)
{
/* !PRESENT -> * ; no need for flush */
if (!(pte_flags(oldpte) & _PAGE_PRESENT))
return PTE_FLUSH_NONE;

/* PFN changed ; needs flush */
- if (pte_pfn(oldpte) != pte_pfn(newpte))
+ if (!check_pfn)
+ VM_BUG_ON(pte_pfn(oldpte) != pte_pfn(newpte));
+ else if (pte_pfn(oldpte) != pte_pfn(newpte))
return PTE_FLUSH_STRICT;

/*
@@ -363,14 +366,17 @@ static inline enum pte_flush_type pte_flush_type(pte_t oldpte, pte_t newpte)
* huge_pmd_flush_type() checks whether permissions were demoted and require a
* flush. It should only be used for userspace huge PMDs.
*/
-static inline enum pte_flush_type huge_pmd_flush_type(pmd_t oldpmd, pmd_t newpmd)
+static inline enum pte_flush_type huge_pmd_flush_type(pmd_t oldpmd, pmd_t newpmd,
+ bool check_pfn)
{
/* !PRESENT -> * ; no need for flush */
if (!(pmd_flags(oldpmd) & _PAGE_PRESENT))
return PTE_FLUSH_NONE;

/* PFN changed ; needs flush */
- if (pmd_pfn(oldpmd) != pmd_pfn(newpmd))
+ if (!check_pfn)
+ VM_BUG_ON(pmd_pfn(oldpmd) != pmd_pfn(newpmd));
+ else if (pmd_pfn(oldpmd) != pmd_pfn(newpmd))
return PTE_FLUSH_STRICT;

/*
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 07b3eb8caf63..aee9da6cc5d5 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -677,14 +677,16 @@ static inline void tlb_flush_p4d_range(struct mmu_gather *tlb,
#endif

#ifndef pte_flush_type
-static inline struct pte_flush_type pte_flush_type(pte_t oldpte, pte_t newpte)
+static inline struct pte_flush_type pte_flush_type(pte_t oldpte, pte_t newpte,
+ bool check_pfn)
{
return PTE_FLUSH_STRICT;
}
#endif

#ifndef huge_pmd_flush_type
-static inline bool huge_pmd_flush_type(pmd_t oldpmd, pmd_t newpmd)
+static inline bool huge_pmd_flush_type(pmd_t oldpmd, pmd_t newpmd,
+ bool check_pfn)
{
return PTE_FLUSH_STRICT;
}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b32b7da0f6f7..92a7b3ca317f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1818,7 +1818,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,

flush_type = PTE_FLUSH_STRICT;
if (!tlb->strict)
- flush_type = huge_pmd_flush_type(oldpmd, entry);
+ flush_type = huge_pmd_flush_type(oldpmd, entry, false);
if (flush_type != PTE_FLUSH_NONE)
tlb_flush_pmd_range(tlb, addr, HPAGE_PMD_SIZE,
flush_type == PTE_FLUSH_STRICT);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index cf775f6c8c08..78081d7f4edf 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -204,7 +204,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,

flush_type = PTE_FLUSH_STRICT;
if (!tlb->strict)
- flush_type = pte_flush_type(oldpte, ptent);
+ flush_type = pte_flush_type(oldpte, ptent, false);
if (flush_type != PTE_FLUSH_NONE)
tlb_flush_pte_range(tlb, addr, PAGE_SIZE,
flush_type == PTE_FLUSH_STRICT);
diff --git a/mm/rmap.c b/mm/rmap.c
index 62f4b2a4f067..63261619b607 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -974,7 +974,7 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
entry = pte_wrprotect(oldpte);
entry = pte_mkclean(entry);

- if (pte_flush_type(oldpte, entry) != PTE_FLUSH_NONE ||
+ if (pte_flush_type(oldpte, entry, false) != PTE_FLUSH_NONE ||
mm_tlb_flush_pending(vma->vm_mm))
flush_tlb_page(vma, address);

--
2.25.1