On 14.02.25 07:32, yangge1116@xxxxxxx wrote:
From: Ge Yang <yangge1116@xxxxxxx>
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.
I wonder if that waiting should instead be done exactly once after
migrating multiple folios? For example, at the beginning of
test_pages_isolated(), to "flush" that state from any previous migration?
Thanks for all your effort around making CMA allocations / migration
more reliable.
--
Cheers,
David / dhildenb
