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

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

On 4/23/26 07:09, Salunke, Hrushikesh wrote:
> 
> On 22-04-2026 23:55, David Hildenbrand (Arm) wrote:
>> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
>>
>>
>> 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
> 
> Right, on x86 the max buddy order keeps it well within safe limits.
> 
> Also, rep stosb/stosq on x86, currently used for clearing, is
> interruptible, the CPU can take interrupts between iterations and
> resume where it left off. 

Ah yes, indeed!

-- 
Cheers,

David