mempolicy: optimize queue_folios_pte_range by PTE batching

After the check for queue_folio_required(), the code only cares about the
folio in the for loop, i.e the PTEs are redundant.  Therefore, optimize
this loop by skipping over a PTE batch mapping the same folio.

With a test program migrating pages of the calling process, which includes
a mapped VMA of size 4GB with pte-mapped large folios of order-9, and
migrating once back and forth node-0 and node-1, the average execution
time reduces from 7.5 to 4 seconds, giving an approx 47% speedup.

Link: https://lkml.kernel.org/r/20250416053048.96479-1-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b28a1e6ae09641ff4bc15bd9fcb329ea60c47374..4d2dc8b6396598e401163c610680697cea0ec97b 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -566,6 +566,7 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
 static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
                        unsigned long end, struct mm_walk *walk)
 {
+       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
        struct vm_area_struct *vma = walk->vma;
        struct folio *folio;
        struct queue_pages *qp = walk->private;
@@ -573,6 +574,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
        pte_t *pte, *mapped_pte;
        pte_t ptent;
        spinlock_t *ptl;
+       int max_nr, nr;
 
        ptl = pmd_trans_huge_lock(pmd, vma);
        if (ptl) {
@@ -586,7 +588,9 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
                walk->action = ACTION_AGAIN;
                return 0;
        }
-       for (; addr != end; pte++, addr += PAGE_SIZE) {
+       for (; addr != end; pte += nr, addr += nr * PAGE_SIZE) {
+               max_nr = (end - addr) >> PAGE_SHIFT;
+               nr = 1;
                ptent = ptep_get(pte);
                if (pte_none(ptent))
                        continue;
@@ -598,6 +602,10 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
                folio = vm_normal_folio(vma, addr, ptent);
                if (!folio || folio_is_zone_device(folio))
                        continue;
+               if (folio_test_large(folio) && max_nr != 1)
+                       nr = folio_pte_batch(folio, addr, pte, ptent,
+                                            max_nr, fpb_flags,
+                                            NULL, NULL, NULL);
                /*
                 * vm_normal_folio() filters out zero pages, but there might
                 * still be reserved folios to skip, perhaps in a VDSO.
@@ -630,7 +638,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
                if (!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
                    !vma_migratable(vma) ||
                    !migrate_folio_add(folio, qp->pagelist, flags)) {
-                       qp->nr_failed++;
+                       qp->nr_failed += nr;
                        if (strictly_unmovable(flags))
                                break;
                }