Re: [PATCH v2] mm/page_alloc: use batch page clearing in kernel_init_pages()

[David Hildenbrand (Arm)]

On 4/21/26 15:44, Zi Yan wrote:
> On 21 Apr 2026, at 0:24, 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.
>>
>> Use it in kernel_init_pages() to replace the per-page loop.
>>
>> 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%
>>
>> Signed-off-by: Hrushikesh Salunke <hsalunke@xxxxxxx>
>> ---
>> base commit: f1541b40cd422d7e22273be9b7e9edfc9ea4f0d7
>>
>> v1: https://lore.kernel.org/all/20260408092441.435133-1-hsalunke@xxxxxxx/
>>
>> 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.
>>
>>  include/linux/highmem.h | 12 ++++++++++++
>>  mm/page_alloc.c         |  5 +----
>>  2 files changed, 13 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index af03db851a1d..ad0f42d06ce6 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -345,6 +345,18 @@ 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)
>> +{
>> +	if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>> +		clear_pages(kasan_reset_tag(page_address(page)), numpages);
> 
> kasan_reset_tag() here removes the tag from page address, so that
> clear_pages() can use the right kaddr. I thought each page needs
> a kasan_reset_tag(). No need to respond here, as I am reading
> the code and trying to understand how it works.

It's all confusing. But we really just turn the pointer into an untagged
pointer here, once.

So I think this is ok.

I do wonder, though, whether we want to move the
kasan_disable_current/kasan_enable_current into the
clear_highpages_kasan_tagged().

-- 
Cheers,

David