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

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

On 08-04-2026 15:17, Vlastimil Babka (SUSE) wrote:

> Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
>
>
> On 4/8/26 11:24, Hrushikesh Salunke wrote:
>> When init_on_alloc is enabled, kernel_init_pages() clears every page
>> one at a time, calling clear_page() per page.  This is unnecessarily
>> slow for large contiguous allocations (mTHPs, HugeTLB) that dominate
>> real workloads.
>>
>> On 64-bit (!HIGHMEM) systems, switch to clearing pages in batch via
>> clear_pages(), bypassing the per-page kmap_local_page()/kunmap_local()
>> overhead and allowing the arch clearing primitive to operate on the full
>> contiguous range in a single invocation.  The batch size is the full
>> allocation when the preempt model is preemptible (preemption points are
>> implicit), or PROCESS_PAGES_NON_PREEMPT_BATCH otherwise, with
>> cond_resched() between batches to limit scheduling latency under
>> cooperative preemption.
>>
>> The HIGHMEM path is kept as-is since those pages require kmap.
>>
>> 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@amd.com>
>> ---
>> base commit: 1a2fbbe3653f0ebb24af9b306a8a968287344a35
> Any way to reuse the code added by [1], e.g. clear_user_highpages()?
>
> [1]
> https://lore.kernel.org/linux-mm/20250917152418.4077386-1-ankur.a.arora@oracle.com/

Thanks for the review. Sure, I will check if code reuse is possible.
Meanwhile I found another issue with the current patch.

kernel_init_pages() runs inside the allocator (post_alloc_hook and
__free_pages_prepare), so it inherits whatever context the caller is in.
Testing with CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_PROVE_LOCKING=y, I
hit this during exit_group() -> exit_mmap() -> __zap_vma_range, where a
page allocation happens while the PTE lock and RCU read lock are held,
making the cond_resched() in the clearing loop illegal:

[ 1997.353228] BUG: sleeping function called from invalid context at mm/page_alloc.c:1235
[ 1997.353433] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 19725, name: bash
[ 1997.353572] preempt_count: 1, expected: 0
[ 1997.353706] RCU nest depth: 1, expected: 0
[ 1997.353837] 3 locks held by bash/19725:
[ 1997.353839]  #0: ff38cd415971e540 (&mm->mmap_lock){++++}-{4:4}, at: exit_mmap+0x6e/0x430
[ 1997.353850]  #1: ffffffffb03d6f60 (rcu_read_lock){....}-{1:3}, at: __pte_offset_map+0x2c/0x220
[ 1997.353855]  #2: ff38cd410deb4618 (ptlock_ptr(ptdesc)#2){+.+.}-{3:3}, at: pte_offset_map_lock+0x92/0x170
[ 1997.353868] Call Trace:
[ 1997.353870]  <TASK>
[ 1997.353873]  dump_stack_lvl+0x91/0xb0
[ 1997.353877]  __might_resched+0x15f/0x290
[ 1997.353882]  kernel_init_pages+0x4b/0xa0
[ 1997.353886]  get_page_from_freelist+0x406/0x1e60
[ 1997.353895]  __alloc_frozen_pages_noprof+0x1d8/0x1730
[ 1997.353912]  alloc_pages_mpol+0xa4/0x190
[ 1997.353917]  alloc_pages_noprof+0x59/0xd0
[ 1997.353919]  get_free_pages_noprof+0x11/0x40
[ 1997.353921]  __tlb_remove_folio_pages_size.isra.0+0x7f/0xe0
[ 1997.353923]  __zap_vma_range+0x1bbd/0x1f40
[ 1997.353931]  unmap_vmas+0xd9/0x1d0
[ 1997.353934]  exit_mmap+0x10a/0x430
[ 1997.353943]  __mmput+0x3d/0x130
[ 1997.353947]  do_exit+0x2a7/0xae0
[ 1997.353951]  do_group_exit+0x36/0xa0
[ 1997.353953]  __x64_sys_exit_group+0x18/0x20
[ 1997.353959]  do_syscall_64+0xe1/0x710
[ 1997.353990]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[ 1997.354003]  </TASK>

This also means clear_contig_highpages() can't be directly reused here
since it has an unconditional might_sleep() + cond_resched(). I'll look
into this. Any suggestions on the right way to handle cond_resched()
in a context that may or may not be atomic?

Thanks,
Hrushikesh

>>  mm/page_alloc.c | 19 +++++++++++++++++--
>>  1 file changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index b1c5430cad4e..178cbebadd50 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1224,8 +1224,23 @@ static void kernel_init_pages(struct page *page, int numpages)
>>
>>       /* s390's use of memset() could override KASAN redzones. */
>>       kasan_disable_current();
>> -     for (i = 0; i < numpages; i++)
>> -             clear_highpage_kasan_tagged(page + i);
>> +
>> +     if (!IS_ENABLED(CONFIG_HIGHMEM)) {
>> +             void *addr = kasan_reset_tag(page_address(page));
>> +             unsigned int unit = preempt_model_preemptible() ?
>> +                                     numpages : PROCESS_PAGES_NON_PREEMPT_BATCH;
>> +             int count;
>> +
>> +             for (i = 0; i < numpages; i += count) {
>> +                     cond_resched();
>> +                     count = min_t(int, unit, numpages - i);
>> +                     clear_pages(addr + (i << PAGE_SHIFT), count);
>> +             }
>> +     } else {
>> +             for (i = 0; i < numpages; i++)
>> +                     clear_highpage_kasan_tagged(page + i);
>> +     }
>> +
>>       kasan_enable_current();
>>  }
>>