Re: [PATCH 2/5] mm: page_alloc: optimize pfn_range_valid_contig()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2026/1/13 9:27, Zi Yan wrote:
> On 12 Jan 2026, at 20:24, Kefeng Wang wrote:
> 
>> On 2026/1/13 1:02, Zi Yan wrote:
>>> On 12 Jan 2026, at 10:09, Kefeng Wang wrote:
>>>
>>>> The alloc_contig_pages() spends a significant amount of time within
>>>> pfn_range_valid_contig().
>>>>
>>>> - set_max_huge_pages
>>>>      - 99.98% alloc_pool_huge_folio
>>>>           only_alloc_fresh_hugetlb_folio.isra.0
>>>>         - alloc_contig_frozen_pages_noprof
>>>>            - 87.00% pfn_range_valid_contig
>>>>                 pfn_to_online_page
>>>>            - 12.91% alloc_contig_frozen_range_noprof
>>>>                 4.51% replace_free_hugepage_folios
>>>>               - 4.02% prep_new_page
>>>>                    prep_compound_page
>>>>               - 2.98% undo_isolate_page_range
>>>>                  - 2.79% unset_migratetype_isolate
>>>>                     - 2.75% __move_freepages_block_isolate
>>>>                          2.71% __move_freepages_block
>>>>               - 0.98% start_isolate_page_range
>>>>                    0.66% set_migratetype_isolate
>>>>
>>>> To optimize this process, use the new helper has_unmovable_pages()
>>>
>>> s/has_unmovable_pages/page_is_unmovable
>>
>> Indeed.
>>
>>>
>>>> to avoid more unnecessary iterations for compound pages, such as
>>>> THP, and high-order buddy pages, which significantly improving the
>>>
>>> s/THP/THP not on LRU/
>>
>> Sure
>>
>>>
>>>> efficiency of contiguous memory allocation.
>>>>
>>>> A simple test on machine with 114G free memory, allocate 120 * 1G
>>>> HugeTLB folios(104 successfully returned),
>>>>
>>>>     time echo 120 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
>>>>
>>>> Before: 0m3.605s
>>>> After:  0m0.602s
>>>>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>>    mm/page_alloc.c | 25 ++++++++-----------------
>>>>    1 file changed, 8 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index d8d5379c44dc..813c5f57883f 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -7157,18 +7157,20 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>>>>    				   unsigned long nr_pages, bool skip_hugetlb,
>>>>    				   bool *skipped_hugetlb)
>>>>    {
>>>> -	unsigned long i, end_pfn = start_pfn + nr_pages;
>>>> +	unsigned long end_pfn = start_pfn + nr_pages;
>>>>    	struct page *page;
>>>>
>>>> -	for (i = start_pfn; i < end_pfn; i++) {
>>>> -		page = pfn_to_online_page(i);
>>>> +	while (start_pfn < end_pfn) {
>>>> +		unsigned long step = 1;
>>>> +
>>>> +		page = pfn_to_online_page(start_pfn);
>>>>    		if (!page)
>>>>    			return false;
>>>>
>>>>    		if (page_zone(page) != z)
>>>>    			return false;
>>>>
>>>> -		if (PageReserved(page))
>>>> +		if (page_is_unmovable(z, page, PB_ISOLATE_MODE_OTHER, &step))
>>>>    			return false;
>>>>
>>>>    		/*
>>>> @@ -7183,9 +7185,6 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>>>>    		if (PageHuge(page)) {
>>>>    			unsigned int order;
>>>>
>>>> -			if (!IS_ENABLED(CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION))
>>>> -				return false;
>>>> -
>>>>    			if (skip_hugetlb) {
>>>>    				*skipped_hugetlb = true;
>>>>    				return false;
>>>> @@ -7196,17 +7195,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>>>>    			if ((order >= MAX_FOLIO_ORDER) ||
>>>>    			    (nr_pages <= (1 << order)))
>>>>    				return false;
>>>
>>> How does page_is_unmovable() interact with the code inside “if (PageHuge(page))”?
>>> page_is_unmovable() only identify 1GB hugetlb as unmovable, so skip_hugetlb still
>>> works?
>>
>> Initially, I wanted to move the skip_hugetlb processing into a new
>> page_is_unmovable() by introducing a new PB_ISOLATE_MODE, passing the
>> skip_hugetlb/skipped_hugetlb/nr_pages to page_is_unmovable(), it looks
>> very complicated/ugly.
>>
>> if (PageHuage()) {
>> 	if(page is unmovable)
>> 		return;
>> 	skip_hugetlb processing
>> }
>>
>> Back to the current code before I made any changes, skip_hugetlb logical
>> only works for movable huge pages by checking
>> CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION, the checking is not incomplete
>> since no runtime check, but the new helper made a better judgment.
>>
>>
>>
>> And after changes,
>>
>> if (page_is_unmovale())
>> 	return
>>
>> if (PageHuge())
>> 	skip_hugetlb processing
>> 	
>> I don' change the skip hugetlb logical, the only drawback is the
>> PageHuge is checked twice, Maybe I miss something?
> 
> Sounds good to me. Thanks. I just want to double check with you.
> 
> With the changes to the commit message, feel free to add

OK, I will update it.

> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> 

Thanks.