[PATCH v2 13/14] arm64/mm: Implement ptep_set_wrprotects() to optimize fork()

Ryan Roberts
2023-11-15T16:31:20Z

With the core-mm changes in place to batch-copy ptes during fork, we can
take advantage of this in arm64 to greatly reduce the number of tlbis we
have to issue, and recover the lost fork performance incured when adding
support for transparent contiguous ptes.

If we are write-protecting a whole contig range, we can apply the
write-protection to the whole range and know that it won't change
whether the range should have the contiguous bit set or not. For ranges
smaller than the contig range, we will still have to unfold, apply the
write-protection, then fold if the change now means the range is
foldable.

This optimization is possible thanks to the tightening of the Arm ARM in
respect to the definition and behaviour when 'Misprogramming the
Contiguous bit'. See section D21194 at
https://developer.arm.com/documentation/102105/latest/

Performance tested with the following test written for the will-it-scale
framework:

-------

char *testcase_description = "fork and exit";

void testcase(unsigned long long *iterations, unsigned long nr)
{
int pid;
char *mem;

mem = malloc(SZ_128M);
assert(mem);
memset(mem, 1, SZ_128M);

while (1) {
pid = fork();
assert(pid >= 0);

if (!pid)
exit(0);

waitpid(pid, NULL, 0);

(*iterations)++;
}
}

-------

I see huge performance regression when PTE_CONT support was added, then
the regression is mostly fixed with the addition of this change. The
following shows regression relative to before PTE_CONT was enabled
(bigger negative value is bigger regression):

| cpus | before opt | after opt |
|-------:|-------------:|------------:|
| 1 | -10.4% | -5.2% |
| 8 | -15.4% | -3.5% |
| 16 | -38.7% | -3.7% |
| 24 | -57.0% | -4.4% |
| 32 | -65.8% | -5.4% |

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
arch/arm64/include/asm/pgtable.h | 30 ++++++++++++++++++++---
arch/arm64/mm/contpte.c | 42 ++++++++++++++++++++++++++++++++
2 files changed, 69 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 15bc9cf1eef4..9bd2f57a9e11 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -984,6 +984,16 @@ static inline void __ptep_set_wrprotect(struct mm_struct *mm,
} while (pte_val(pte) != pte_val(old_pte));
}

+static inline void __ptep_set_wrprotects(struct mm_struct *mm,
+ unsigned long address, pte_t *ptep,
+ unsigned int nr)
+{
+ unsigned int i;
+
+ for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++)
+ __ptep_set_wrprotect(mm, address, ptep);
+}
+
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
#define __HAVE_ARCH_PMDP_SET_WRPROTECT
static inline void pmdp_set_wrprotect(struct mm_struct *mm,
@@ -1139,6 +1149,8 @@ extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep);
extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep);
+extern void contpte_set_wrprotects(struct mm_struct *mm, unsigned long addr,
+ pte_t *ptep, unsigned int nr);
extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep,
pte_t entry, int dirty);
@@ -1290,13 +1302,25 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
return contpte_ptep_clear_flush_young(vma, addr, ptep);
}

+#define ptep_set_wrprotects ptep_set_wrprotects
+static inline void ptep_set_wrprotects(struct mm_struct *mm, unsigned long addr,
+ pte_t *ptep, unsigned int nr)
+{
+ if (!contpte_is_enabled(mm))
+ __ptep_set_wrprotects(mm, addr, ptep, nr);
+ else if (nr == 1) {
+ contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+ __ptep_set_wrprotects(mm, addr, ptep, 1);
+ contpte_try_fold(mm, addr, ptep, __ptep_get(ptep));
+ } else
+ contpte_set_wrprotects(mm, addr, ptep, nr);
+}
+
#define __HAVE_ARCH_PTEP_SET_WRPROTECT
static inline void ptep_set_wrprotect(struct mm_struct *mm,
unsigned long addr, pte_t *ptep)
{
- contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
- __ptep_set_wrprotect(mm, addr, ptep);
- contpte_try_fold(mm, addr, ptep, __ptep_get(ptep));
+ ptep_set_wrprotects(mm, addr, ptep, 1);
}

#define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 667bcf7c3260..426be9cd4dea 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -302,6 +302,48 @@ int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
}
EXPORT_SYMBOL(contpte_ptep_clear_flush_young);

+void contpte_set_wrprotects(struct mm_struct *mm, unsigned long addr,
+ pte_t *ptep, unsigned int nr)
+{
+ unsigned long next;
+ unsigned long end = addr + (nr << PAGE_SHIFT);
+
+ do {
+ next = pte_cont_addr_end(addr, end);
+ nr = (next - addr) >> PAGE_SHIFT;
+
+ /*
+ * If wrprotecting an entire contig range, we can avoid
+ * unfolding. Just set wrprotect and wait for the later
+ * mmu_gather flush to invalidate the tlb. Until the flush, the
+ * page may or may not be wrprotected. After the flush, it is
+ * guarranteed wrprotected. If its a partial range though, we
+ * must unfold, because we can't have a case where CONT_PTE is
+ * set but wrprotect applies to a subset of the PTEs; this would
+ * cause it to continue to be unpredictable after the flush.
+ */
+ if (nr != CONT_PTES)
+ contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
+
+ __ptep_set_wrprotects(mm, addr, ptep, nr);
+
+ addr = next;
+ ptep += nr;
+
+ /*
+ * If applying to a partial contig range, the change could have
+ * made the range foldable. Use the last pte in the range we
+ * just set for comparison, since contpte_try_fold() only
+ * triggers when acting on the last pte in the contig range.
+ */
+ if (nr != CONT_PTES)
+ contpte_try_fold(mm, addr - PAGE_SIZE, ptep - 1,
+ __ptep_get(ptep - 1));
+
+ } while (addr != end);
+}
+EXPORT_SYMBOL(contpte_set_wrprotects);
+
int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep,
pte_t entry, int dirty)
--
2.25.1