mm/page_alloc: replace kernel_init_pages() with batch page clearing

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%

[hsalunke@amd.com: move clear_highpages_kasan_tagged() to page_alloc.c]
  Link: https://lore.kernel.org/20260504063942.553438-1-hsalunke@amd.com
Link: https://lore.kernel.org/20260422102729.166599-1-hsalunke@amd.com
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>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shivank Garg <shivankg@amd.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 11 insertions, 7 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d49c254174da..bf53242d3db7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1211,14 +1211,18 @@ 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();
 }
 
@@ -1423,7 +1427,7 @@ __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
@@ -1848,7 +1852,7 @@ 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);