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

["Salunke, Hrushikesh" <hsalunke@amd.com>]

On 23-04-2026 16:42, Andrew Morton wrote:
> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
>
>
> On Wed, 22 Apr 2026 10:26:58 +0000 Hrushikesh Salunke <hsalunke@amd.com> 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?
>
Hi Andrew,

The idea was to keep it alongside clear_highpage_kasan_tagged() as its
batch counterpart, but currently it is only used by page_alloc.c.

Your concern about the code size increase is valid. Would you prefer if
I move it to page_alloc.c as a static function and drop the inline
in v4? If an external user comes along later it can always be moved
back to the header.

regards,
Hrushikesh