Let's implement PTE batching when consecutive (present) PTEs map
consecutive pages of the same large folio, and all other PTE bits besides
the PFNs are equal.
We will optimize folio_pte_batch() separately, to ignore selected
PTE bits. This patch is based on work by Ryan Roberts.
Use __always_inline for __copy_present_ptes() and keep the handling for
single PTEs completely separate from the multi-PTE case: we really want
the compiler to optimize for the single-PTE case with small folios, to
not degrade performance.
Note that PTE batching will never exceed a single page table and will
always stay within VMA boundaries.
Further, processing PTE-mapped THP that maybe pinned and have
PageAnonExclusive set on at least one subpage should work as expected,
but there is room for improvement: We will repeatedly (1) detect a PTE
batch (2) detect that we have to copy a page (3) fall back and allocate a
single page to copy a single page. For now we won't care as pinned pages
are a corner case, and we should rather look into maintaining only a
single PageAnonExclusive bit for large folios.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
include/linux/pgtable.h | 31 +++++++++++
mm/memory.c | 112 +++++++++++++++++++++++++++++++++-------
2 files changed, 124 insertions(+), 19 deletions(-)
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 351cd9dc7194f..891ed246978a4 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
}
#endif
+#ifndef wrprotect_ptes
+/**
+ * wrprotect_ptes - Write-protect consecutive pages that are mapped to a
+ * contiguous range of addresses.
+ * @mm: Address space to map the pages into.
+ * @addr: Address the first page is mapped at.
+ * @ptep: Page table pointer for the first entry.
+ * @nr: Number of pages to write-protect.
+ *
+ * May be overridden by the architecture; otherwise, implemented as a simple
+ * loop over ptep_set_wrprotect().
+ *
+ * Note that PTE bits in the PTE range besides the PFN can differ. For example,
+ * some PTEs might already be write-protected.
+ *
+ * Context: The caller holds the page table lock. The pages all belong
+ * to the same folio. The PTEs are all in the same PMD.
+ */
+static inline void wrprotect_ptes(struct mm_struct *mm, unsigned long addr,
+ pte_t *ptep, unsigned int nr)
+{
+ for (;;) {
+ ptep_set_wrprotect(mm, addr, ptep);
+ if (--nr == 0)
+ break;
+ ptep++;
+ addr += PAGE_SIZE;
+ }
+}
+#endif
+
/*
* On some architectures hardware does not set page access bit when accessing
* memory page, it is responsibility of software setting this bit. It brings
diff --git a/mm/memory.c b/mm/memory.c
index 729ca4d6a820c..4d1be89a01ee0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -930,15 +930,15 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
return 0;
}
-static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
+static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte,
- pte_t pte, unsigned long addr)
+ pte_t pte, unsigned long addr, int nr)
{
struct mm_struct *src_mm = src_vma->vm_mm;
/* If it's a COW mapping, write protect it both processes. */
if (is_cow_mapping(src_vma->vm_flags) && pte_write(pte)) {
- ptep_set_wrprotect(src_mm, addr, src_pte);
+ wrprotect_ptes(src_mm, addr, src_pte, nr);
pte = pte_wrprotect(pte);
}
@@ -950,26 +950,93 @@ static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
if (!userfaultfd_wp(dst_vma))
pte = pte_clear_uffd_wp(pte);
- set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
+ set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
+}
+
+/*
+ * Detect a PTE batch: consecutive (present) PTEs that map consecutive
+ * pages of the same folio.
+ *
+ * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN.
+ */
+static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
+ pte_t *start_ptep, pte_t pte, int max_nr)
+{
+ unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
+ const pte_t *end_ptep = start_ptep + max_nr;
+ pte_t expected_pte = pte_next_pfn(pte);
+ pte_t *ptep = start_ptep + 1;
+
+ VM_WARN_ON_FOLIO(!pte_present(pte), folio);
+
+ while (ptep != end_ptep) {
+ pte = ptep_get(ptep);
+
+ if (!pte_same(pte, expected_pte))
+ break;
+
+ /*
+ * Stop immediately once we reached the end of the folio. In
+ * corner cases the next PFN might fall into a different
+ * folio.
+ */
+ if (pte_pfn(pte) == folio_end_pfn)
+ break;
+
+ expected_pte = pte_next_pfn(expected_pte);
+ ptep++;
+ }
+
+ return ptep - start_ptep;
}
/*
- * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page
- * is required to copy this pte.
+ * Copy one present PTE, trying to batch-process subsequent PTEs that map
+ * consecutive pages of the same folio by copying them as well.
+ *
+ * Returns -EAGAIN if one preallocated page is required to copy the next PTE.
+ * Otherwise, returns the number of copied PTEs (at least 1).
*/
static inline int
-copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
+copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
pte_t *dst_pte, pte_t *src_pte, pte_t pte, unsigned long addr,
- int *rss, struct folio **prealloc)
+ int max_nr, int *rss, struct folio **prealloc)
{
struct page *page;
struct folio *folio;
+ int err, nr;
page = vm_normal_page(src_vma, addr, pte);
if (unlikely(!page))
goto copy_pte;
folio = page_folio(page);
+
+ /*
+ * If we likely have to copy, just don't bother with batching. Make
+ * sure that the common "small folio" case is as fast as possible
+ * by keeping the batching logic separate.
+ */
+ if (unlikely(!*prealloc && folio_test_large(folio) && max_nr != 1)) {
+ nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr);
+ folio_ref_add(folio, nr);
+ if (folio_test_anon(folio)) {
+ if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
+ nr, src_vma))) {
+ folio_ref_sub(folio, nr);
+ return -EAGAIN;
+ }
+ rss[MM_ANONPAGES] += nr;
+ VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio);
+ } else {
+ folio_dup_file_rmap_ptes(folio, page, nr);
+ rss[mm_counter_file(page)] += nr;
+ }
+ __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte,
+ addr, nr);
+ return nr;
+ }
+
folio_get(folio);
if (folio_test_anon(folio)) {
/*
@@ -981,8 +1048,9 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) {
/* Page may be pinned, we have to copy. */
folio_put(folio);
- return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
- addr, rss, prealloc, page);
+ err = copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
+ addr, rss, prealloc, page);
+ return err ? err : 1;
}
rss[MM_ANONPAGES]++;
VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio);
@@ -992,8 +1060,8 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
}
copy_pte:
- __copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, pte, addr);
- return 0;
+ __copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte, pte, addr, 1);
+ return 1;
}
static inline struct folio *folio_prealloc(struct mm_struct *src_mm,
@@ -1030,10 +1098,11 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
pte_t *src_pte, *dst_pte;
pte_t ptent;
spinlock_t *src_ptl, *dst_ptl;
- int progress, ret = 0;
+ int progress, max_nr, ret = 0;
int rss[NR_MM_COUNTERS];
swp_entry_t entry = (swp_entry_t){0};
struct folio *prealloc = NULL;
+ int nr;
again:
progress = 0;
@@ -1064,6 +1133,8 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
arch_enter_lazy_mmu_mode();
do {
+ nr = 1;
+
/*
* We are holding two locks at this point - either of them
* could generate latencies in another task on another CPU.
@@ -1100,9 +1171,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
*/
WARN_ON_ONCE(ret != -ENOENT);
}
- /* copy_present_pte() will clear `*prealloc' if consumed */
- ret = copy_present_pte(dst_vma, src_vma, dst_pte, src_pte,
- ptent, addr, rss, &prealloc);
+ /* copy_present_ptes() will clear `*prealloc' if consumed */
+ max_nr = (end - addr) / PAGE_SIZE;
+ ret = copy_present_ptes(dst_vma, src_vma, dst_pte, src_pte,
+ ptent, addr, max_nr, rss, &prealloc);
/*
* If we need a pre-allocated page for this pte, drop the
* locks, allocate, and try again.
@@ -1119,8 +1191,10 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
folio_put(prealloc);
prealloc = NULL;
}
- progress += 8;
- } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
+ nr = ret;
+ progress += 8 * nr;
+ } while (dst_pte += nr, src_pte += nr, addr += PAGE_SIZE * nr,
+ addr != end);
arch_leave_lazy_mmu_mode();
pte_unmap_unlock(orig_src_pte, src_ptl);
@@ -1141,7 +1215,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
prealloc = folio_prealloc(src_mm, src_vma, addr, false);
if (!prealloc)
return -ENOMEM;
- } else if (ret) {
+ } else if (ret < 0) {
VM_WARN_ON_ONCE(1);
}
--
2.43.0
