Re: [PATCH v3] mm/page_alloc: replace kernel_init_pages() with batch page clearing

[Andrew Morton]

On Wed, 22 Apr 2026 10:26:58 +0000 Hrushikesh Salunke <hsalunke@xxxxxxx> 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() 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.
> 
> 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)

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%
> 
> ...
>
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -345,6 +345,21 @@ 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)
> +{
> +	/* s390's use of memset() could override KASAN redzones. */
> +	kasan_disable_current();
> +	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();
> +}

Why was it globally published and inlined?  Is there any expectation
that this will be used outside of page_alloc.c?

Both of the callsites are themselves inlined.  The patch adds 330 bytes
to my arm allmodcnfig page_alloc.o - did we gain anything from that?