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

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, May 04, 2026 at 06:39:19AM +0000, Hrushikesh Salunke wrote:
> 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)

Wow nice!

>
> 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%

Lovely :)

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

All looks sensible to me so:

Acked-by: Lorenzo Stoakes <ljs@kernel.org>

Cheers, Lorenzo

> 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)) {

I hope that, soon, we won't need this :)

> +		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
>