On 19 Nov 2024, at 10:29, David Hildenbrand wrote:
>> +/* Split a multi-block free page into its individual pageblocks. */
>> +static void split_large_buddy(struct zone *zone, struct page *page,
>> + unsigned long pfn, int order, fpi_t fpi)
>> +{
>> + unsigned long end = pfn + (1 << order);
>> +
>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn, 1 << order));
>> + /* Caller removed page from freelist, buddy info cleared! */
>> + VM_WARN_ON_ONCE(PageBuddy(page));
>> +
>> + if (order > pageblock_order)
>> + order = pageblock_order;
>> +
>> + while (pfn != end) {
>> + int mt = get_pfnblock_migratetype(page, pfn);
>> +
>> + __free_one_page(page, pfn, zone, order, mt, fpi);
>> + pfn += 1 << order;
>> + page = pfn_to_page(pfn);
>> + }
>> +}
>
> Hi,
>
> stumbling over this while digging through the code ....
>
>> +
>> static void free_one_page(struct zone *zone, struct page *page,
>> unsigned long pfn, unsigned int order,
>> fpi_t fpi_flags)
>> {
>> unsigned long flags;
>> - int migratetype;
>> spin_lock_irqsave(&zone->lock, flags);
>> - migratetype = get_pfnblock_migratetype(page, pfn);
>> - __free_one_page(page, pfn, zone, order, migratetype, fpi_flags);
>
> This change is rather undesired:
>
> via __free_pages_core()->__free_pages_ok() we can easily end up here with order=MAX_PAGE_ORDER.
Do you have a concrete example? PMD THP on x86_64 is pageblock_order.
We do not have PMD level mTHP yet. Any other possible source?
>
> What your new code will do is split this perfectly reasonable MAX_PAGE_ORDER chunk via split_large_buddy() into pageblock-sized chunks, and let the buddy merging logic undo our unnecessary splitting.
>
> Is there a way to avoid this and just process the whole MAX_PAGE_ORDER chunk like we used to?
Probably split_large_buddy() can check the migratetypes of the to-be-freed
page, if order > pageblock_order. If all migratetypes are the same, the page
can be freed at MAX_PAGE_ORDER, otherwise pageblock_order.
Best Regards,
Yan, Zi
