[PATCH v10 25/37] mm: use __GFP_ZERO in alloc_anon_folio

[Michael S. Tsirkin]

Convert alloc_anon_folio() to pass __GFP_ZERO instead of zeroing
at the callsite. post_alloc_hook uses the fault address passed
through vma_alloc_folio for cache-friendly zeroing.

Note: before this series, replacing clear_user_highpage() with
__GFP_ZERO was unsafe on cache-aliasing architectures because
__GFP_ZERO uses clear_page() without a dcache flush. With this
series, it is safe if the caller passes a valid user address
(not USER_ADDR_NONE) to vma_alloc_folio() etc., which delivers
it to post_alloc_hook() for the dcache flush via
folio_zero_user(). It is only unsafe if USER_ADDR_NONE is passed.

Note: with __GFP_ZERO, the folio is zeroed before
mem_cgroup_charge().  If the charge fails, the zeroing work is
wasted.  Previously zeroing was done after a successful charge.
This is inherent to moving zeroing into the allocator.
Charge failures are rare (only at cgroup limits).

Use folio_put_zeroed() on charge failure so the zeroed hint
propagates to the buddy allocator, avoiding redundant re-zeroing
on the next allocation attempt.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 mm/memory.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6c14b90f558e..6d6a3e1a02c1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5265,25 +5265,16 @@ 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_ZERO;
 	while (orders) {
 		folio = vma_alloc_folio(gfp, order, vma, vmf->address);
 		if (folio) {
 			if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
 				count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
-				folio_put(folio);
+				folio_put_zeroed(folio);
 				goto next;
 			}
 			folio_throttle_swaprate(folio, gfp);
-			/*
-			 * When a folio is not zeroed during allocation
-			 * (__GFP_ZERO not used) or user folios require special
-			 * handling, folio_zero_user() is used to make sure
-			 * that the page corresponding to the faulting address
-			 * will be hot in the cache after zeroing.
-			 */
-			if (user_alloc_needs_zeroing())
-				folio_zero_user(folio, vmf->address);
 			return folio;
 		}
 next:
-- 
MST