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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/22/26 12:26, 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() 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)
> 
> 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%

We do have some elaborate handling in clear_contig_highpages() to chunk it up
(and to call cond_resched()). But that function can get called with much bigger
ranges.

I'm not concerned about the cond_resched() -- we wouldn't do one here before --
but I'm wondering whether we could end up triggering a HW instruction that is
uninterruptible and takes a rather long time.

But clear_contig_highpages() breaks it into 32MiB chunks, and only x86 supports
it so far. So we won't exceed that with the maximum buddy order of 4MiB on x86.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David