[PATCH RFC v3 04/19] mm: use folio_zero_user for user pages in post_alloc_hook

Michael S. Tsirkin
2026-04-21T22:01:30Z

When post_alloc_hook() needs to zero a page for an explicit
__GFP_ZERO allocation and user_addr is set, use folio_zero_user()
instead of kernel_init_pages(). This zeros near the faulting
address last, keeping those cachelines hot for the impending
user access.

folio_zero_user() is only used for explicit __GFP_ZERO, not for
init_on_alloc. On architectures with virtually-indexed caches
(e.g., ARM), clear_user_highpage() performs per-line cache
operations; using it for init_on_alloc would add overhead that
kernel_init_pages() avoids (the page fault path flushes the
cache at PTE installation time regardless).

No functional change yet: current callers do not pass __GFP_ZERO
for user pages (they zero at the callsite instead). Subsequent
patches will convert them.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
---
mm/page_alloc.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 99c01eb2d59e..db2192ffc27c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1882,9 +1882,20 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
for (i = 0; i != 1 << order; ++i)
page_kasan_tag_reset(page + i);
}
- /* If memory is still not initialized, initialize it now. */
- if (init)
- kernel_init_pages(page, 1 << order);
+ /*
+ * If memory is still not initialized, initialize it now.
+ * When __GFP_ZERO was explicitly requested and user_addr is set,
+ * use folio_zero_user() which zeros near the faulting address
+ * last, keeping those cachelines hot. For init_on_alloc, use
+ * kernel_init_pages() to avoid unnecessary cache flush overhead
+ * on architectures with virtually-indexed caches.
+ */
+ if (init) {
+ if ((gfp_flags & __GFP_ZERO) && user_addr != USER_ADDR_NONE)
+ folio_zero_user(page_folio(page), user_addr);
+ else
+ kernel_init_pages(page, 1 << order);
+ }

set_page_owner(page, order, gfp_flags);
page_table_check_alloc(page, order);
--
MST