[PATCH RFC 5/9] mm: skip zeroing in alloc_anon_folio for pre-zeroed pages

[Michael S. Tsirkin]

Use __GFP_PREZEROED and folio_test_clear_prezeroed() to skip
folio_zero_user() in the mTHP anonymous page allocation path
when the page is already zeroed.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 mm/memory.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 07778814b4a8..2f61321a81fd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5176,7 +5176,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
 		goto fallback;
 
 	/* Try allocating the highest of the remaining orders. */
-	gfp = vma_thp_gfp_mask(vma);
+	gfp = vma_thp_gfp_mask(vma) | __GFP_PREZEROED;
 	while (orders) {
 		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
 		folio = vma_alloc_folio(gfp, order, vma, addr);
@@ -5194,7 +5194,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
 			 * that the page corresponding to the faulting address
 			 * will be hot in the cache after zeroing.
 			 */
-			if (user_alloc_needs_zeroing())
+			if (user_alloc_needs_zeroing() &&
+			    !folio_test_clear_prezeroed(folio))
 				folio_zero_user(folio, vmf->address);
 			return folio;
 		}
-- 
MST