[PATCH v5 02/25] mm: thp: Batch-collapse PMD with set_ptes()

Ryan Roberts
2024-02-02T08:08:24Z

Refactor __split_huge_pmd_locked() so that a present PMD can be
collapsed to PTEs in a single batch using set_ptes().

This should improve performance a little bit, but the real motivation is
to remove the need for the arm64 backend to have to fold the contpte
entries. Instead, since the ptes are set as a batch, the contpte blocks
can be initially set up pre-folded (once the arm64 contpte support is
added in the next few patches). This leads to noticeable performance
improvement during split.

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
mm/huge_memory.c | 58 +++++++++++++++++++++++++++---------------------
1 file changed, 33 insertions(+), 25 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 016e20bd813e..14888b15121e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2579,15 +2579,16 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,

pte = pte_offset_map(&_pmd, haddr);
VM_BUG_ON(!pte);
- for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
- pte_t entry;
- /*
- * Note that NUMA hinting access restrictions are not
- * transferred to avoid any possibility of altering
- * permissions across VMAs.
- */
- if (freeze || pmd_migration) {
+
+ /*
+ * Note that NUMA hinting access restrictions are not transferred to
+ * avoid any possibility of altering permissions across VMAs.
+ */
+ if (freeze || pmd_migration) {
+ for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+ pte_t entry;
swp_entry_t swp_entry;
+
if (write)
swp_entry = make_writable_migration_entry(
page_to_pfn(page + i));
@@ -2606,25 +2607,32 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
entry = pte_swp_mksoft_dirty(entry);
if (uffd_wp)
entry = pte_swp_mkuffd_wp(entry);
- } else {
- entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
- if (write)
- entry = pte_mkwrite(entry, vma);
- if (!young)
- entry = pte_mkold(entry);
- /* NOTE: this may set soft-dirty too on some archs */
- if (dirty)
- entry = pte_mkdirty(entry);
- if (soft_dirty)
- entry = pte_mksoft_dirty(entry);
- if (uffd_wp)
- entry = pte_mkuffd_wp(entry);
+
+ VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+ set_pte_at(mm, addr, pte + i, entry);
}
- VM_BUG_ON(!pte_none(ptep_get(pte)));
- set_pte_at(mm, addr, pte, entry);
- pte++;
+ } else {
+ pte_t entry;
+
+ entry = mk_pte(page, READ_ONCE(vma->vm_page_prot));
+ if (write)
+ entry = pte_mkwrite(entry, vma);
+ if (!young)
+ entry = pte_mkold(entry);
+ /* NOTE: this may set soft-dirty too on some archs */
+ if (dirty)
+ entry = pte_mkdirty(entry);
+ if (soft_dirty)
+ entry = pte_mksoft_dirty(entry);
+ if (uffd_wp)
+ entry = pte_mkuffd_wp(entry);
+
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ VM_WARN_ON(!pte_none(ptep_get(pte + i)));
+
+ set_ptes(mm, haddr, pte, entry, HPAGE_PMD_NR);
}
- pte_unmap(pte - 1);
+ pte_unmap(pte);

if (!pmd_migration)
folio_remove_rmap_pmd(folio, page, vma);
--
2.25.1