[PATCH v4] mm/page_alloc: replace kernel_init_pages() with batch page clearing

[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() as a static batch clearing
helper in page_alloc.c 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.

Replace kernel_init_pages() with direct calls to the new helper, as it
becomes a trivial wrapper.

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>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Pankaj Gupta <pankaj.gupta@amd.com>
---
Hi Andrew,

This is v4 of the batch page clearing patch. v3 is already in
mm-unstable, please replace it with this one.
The only change is moving clear_highpages_kasan_tagged() from
include/linux/highmem.h to mm/page_alloc.c as a static function,
addressing the code size concern you raised on ARM allmodconfig.

Thanks,
Hrushikesh

base commit: 2bcc13c29c711381d815c1ba5d5b25737400c71a

v3: https://lore.kernel.org/all/20260422102729.166599-1-hsalunke@amd.com/
v2: https://lore.kernel.org/all/20260421042451.76918-1-hsalunke@amd.com/
v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@amd.com/

Changes since v3:
- Moved clear_highpages_kasan_tagged() from include/linux/highmem.h to
  mm/page_alloc.c as a static function to avoid code size increase. As
  the function is only used within page_alloc.c.

Changes since v2:
- Moved kasan_disable_current()/kasan_enable_current() into
  clear_highpages_kasan_tagged(), per David and Zi Yan's suggestion.
- Removed kernel_init_pages() and replaced its two call sites with
  direct calls to the helper.

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.

 mm/page_alloc.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 65e205111553..3a59577f58a5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1208,14 +1208,18 @@ static inline bool should_skip_kasan_poison(struct page *page)
 	return page_kasan_tag(page) == KASAN_TAG_KERNEL;
 }
 
-static void kernel_init_pages(struct page *page, int numpages)
+static void clear_highpages_kasan_tagged(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);
+	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);
+	}
 	kasan_enable_current();
 }
 
@@ -1428,7 +1432,7 @@ __always_inline bool __free_pages_prepare(struct page *page,
 			init = false;
 	}
 	if (init)
-		kernel_init_pages(page, 1 << order);
+		clear_highpages_kasan_tagged(page, 1 << order);
 
 	/*
 	 * arch_free_page() can make the page's contents inaccessible.  s390
@@ -1853,7 +1857,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 	}
 	/* If memory is still not initialized, initialize it now. */
 	if (init)
-		kernel_init_pages(page, 1 << order);
+		clear_highpages_kasan_tagged(page, 1 << order);
 
 	set_page_owner(page, order, gfp_flags);
 	page_table_check_alloc(page, order);
-- 
2.43.0