Re: [PATCH 3/5] mm: hugetlb: optimize replace_free_hugepage_folios()

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

On 2026/1/13 13:27, Oscar Salvador wrote:
> On Mon, Jan 12, 2026 at 11:09:52PM +0800, Kefeng Wang wrote:
>> If no free hugepage folios are available, there is no need to perform
>> any replacement operations. Additionally, gigantic folios should not
>> be replaced under any circumstances. Therefore, we only check for the
>> presence of non-gigantic folios.
>>
>> To ensure that gigantic folios are not mistakenly replaced, we utilize
>> isolate_or_dissolve_huge_folio().
>>
>> Lastly, to optimize performance, we skip unnecessary iterations over pfn
>> for compound pages and high-order buddy pages to save processing time.
>>
>> 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: 0m0.602s
>> After:  0m0.431s
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ...
>>   int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn)
>>   {
>> -	struct folio *folio;
>> -	int ret = 0;
>> +	unsigned long nr = 0;
>> +	struct page *page;
>> +	struct hstate *h;
>> +	LIST_HEAD(list);
>>   
>> -	LIST_HEAD(isolate_list);
>> +	/* Avoid pfn iterations if no free non-gigantic huge pages */
>> +	for_each_hstate(h) {
>> +		if (hstate_is_gigantic(h))
>> +			continue;
>> +
>> +		nr += h->free_huge_pages;
>> +		if (nr)
>> +			break;
>> +	}
>> +
>> +	if (!nr)
>> +		return 0;
>>   
>>   	while (start_pfn < end_pfn) {
>> -		folio = pfn_folio(start_pfn);
>> +		page = pfn_to_page(start_pfn);
>> +		nr = 1;
>>   
>> -		/* Not to disrupt normal path by vainly holding hugetlb_lock */
>> -		if (folio_test_hugetlb(folio) && !folio_ref_count(folio)) {
>> -			ret = alloc_and_dissolve_hugetlb_folio(folio, &isolate_list);
>> -			if (ret)
>> -				break;
>> +		if (PageHuge(page) || PageCompound(page)) {
>> +			struct folio *folio = page_folio(page);
>> +
>> +			nr = folio_nr_pages(folio) - folio_page_idx(folio, page);
>> +
>> +			/* Not to disrupt normal path by vainly holding hugetlb_lock */
>> +			if (folio_test_hugetlb(folio) && !folio_ref_count(folio)) {
>> +				if (isolate_or_dissolve_huge_folio(folio, &list))
> 
> isolate_or_dissolve_huge_folio() can also return -EBUSY, but you supersed that with
> ENOMEM in case it fails. Is that alright?> Maybe it is because down the chain we ignore the type of error as 
long as it is
> an error but just brining it up because the log now differs.

The caller(i.e. virtio_mem_fake_offline) may perform some operations
based on different error codes, I will restore this one.

Thanks.