在 2025/2/18 16:55, David Hildenbrand 写道:
On 15.02.25 06:50, Ge Yang wrote:
在 2025/2/14 16:08, David Hildenbrand 写道:
On 14.02.25 07:32, yangge1116@126.com wrote:
From: Ge Yang yangge1116@126.com
Since the introduction of commit b65d4adbc0f0 ("mm: hugetlb: defer freeing of HugeTLB pages"), which supports deferring the freeing of HugeTLB pages, the allocation of contiguous memory through cma_alloc() may fail probabilistically.
In the CMA allocation process, if it is found that the CMA area is occupied by in-use hugepage folios, these in-use hugepage folios need to be migrated to another location. When there are no available hugepage folios in the free HugeTLB pool during the migration of in-use HugeTLB pages, new folios are allocated from the buddy system. A temporary state is set on the newly allocated folio. Upon completion of the hugepage folio migration, the temporary state is transferred from the new folios to the old folios. Normally, when the old folios with the temporary state are freed, it is directly released back to the buddy system. However, due to the deferred freeing of HugeTLB pages, the PageBuddy() check fails, ultimately leading to the failure of cma_alloc().
Here is a simplified call trace illustrating the process: cma_alloc() ->__alloc_contig_migrate_range() // Migrate in-use hugepage ->unmap_and_move_huge_page() ->folio_putback_hugetlb() // Free old folios ->test_pages_isolated() ->__test_page_isolated_in_pageblock() ->PageBuddy(page) // Check if the page is in buddy
To resolve this issue, we have implemented a function named wait_for_hugepage_folios_freed(). This function ensures that the hugepage folios are properly released back to the buddy system after their migration is completed. By invoking wait_for_hugepage_folios_freed() following the migration process, we guarantee that when test_pages_isolated() is executed, it will successfully pass.
Okay, so after every successful migration -> put of src, we wait for the src to actually get freed.
When migrating multiple hugetlb folios, we'd wait once per folio.
It reminds me a bit about pcp caches, where folios are !buddy until the pcp was drained.
It seems that we only track unmovable, reclaimable, and movable pages on the pcp lists. For specific details, please refer to the free_frozen_pages() function.
It reminded me about PCP caches, because we effectively also have to wait for some stuck folios to properly get freed to the buddy.
It seems that when an isolated page is freed, it won't be placed back into the PCP caches.