[PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[Hrushikesh Salunke <hsalunke@amd.com>]

When init_on_alloc is enabled, kernel_init_pages() clears every page
one at a time via clear_highpage_kasan_tagged(), which incurs per-page
kmap_local_page()/kunmap_local() overhead and prevents the architecture
clearing primitive from operating on contiguous ranges.

Introduce clear_highpages_kasan_tagged() in highmem.h, a batch
clearing helper that calls clear_pages() for the full contiguous range
on !HIGHMEM systems, bypassing the per-page kmap overhead and allowing
a single invocation of the arch clearing primitive across the entire
allocation. The HIGHMEM path falls back to per-page clearing since
those pages require kmap.

Use it in kernel_init_pages() to replace the per-page loop.

Allocating 8192 x 2MB HugeTLB pages (16GB) with init_on_alloc=1:

  Before: 0.445s
  After:  0.166s  (-62.7%, 2.68x faster)

Kernel time (sys) reduction per workload with init_on_alloc=1:

  Workload            Before       After       Change
  Graph500 64C128T    30m 41.8s    15m 14.8s   -50.3%
  Graph500 16C32T     15m 56.7s     9m 43.7s   -39.0%
  Pagerank 32T         1m 58.5s     1m 12.8s   -38.5%
  Pagerank 128T        2m 36.3s     1m 40.4s   -35.7%

Signed-off-by: Hrushikesh Salunke <hsalunke@amd.com>
---
base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7

v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/

Changes since v1:
- Dropped cond_resched() and PROCESS_PAGES_NON_PREEMPT_BATCH as
  kernel_init_pages() runs inside the page allocator and can be
  called from atomic context, making cond_resched() unsafe. The 
  original code never had a cond_resched() here, and the 
  performance gain comes from batching, not rescheduling.

- Moved the !HIGHMEM/HIGHMEM branching into a new
  clear_highpages_kasan_tagged() helper in highmem.h, per David's
  suggestion.

 include/linux/highmem.h | 12 ++++++++++++
 mm/page_alloc.c         |  5 +----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index af03db851a1d..ad0f42d06ce6 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -345,6 +345,18 @@ static inline void clear_highpage_kasan_tagged(struct page *page)
 	kunmap_local(kaddr);
 }
 
+static inline void clear_highpages_kasan_tagged(struct page *page, int numpages)
+{
+	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
+		clear_pages(kasan_reset_tag(page_address(page)), numpages);
+	} else {
+		int i;
+
+		for (i = 0; i < numpages; i++)
+			clear_highpage_kasan_tagged(page + i);
+	}
+}
+
 #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGES
 
 /* Return false to let people know we did not initialize the pages */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b1c5430cad4e..1aaf7f839ff4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1220,12 +1220,9 @@ static inline bool should_skip_kasan_poison(struct page *page)
 
 static void kernel_init_pages(struct page *page, int numpages)
 {
-	int i;
-
 	/* s390's use of memset() could override KASAN redzones. */
 	kasan_disable_current();
-	for (i = 0; i < numpages; i++)
-		clear_highpage_kasan_tagged(page + i);
+	clear_highpages_kasan_tagged(page, numpages);
 	kasan_enable_current();
 }
 
-- 
2.43.0