mm/hugetlb: wait for hugetlb folios to be freed

Since the introduction of commit c77c0a8 ("mm/hugetlb: defer freeing
of huge pages if in non-task context"), 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 hugetlb folios, these in-use hugetlb folios need to be
migrated to another location.  When there are no available hugetlb folios
in the free hugetlb pool during the migration of in-use hugetlb folios,
new folios are allocated from the buddy system.  A temporary state is set
on the newly allocated folio.  Upon completion of the hugetlb 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 hugetlb folios
        ->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_freed_hugetlb_folios().  This function ensures that the hugetlb
folios are properly released back to the buddy system after their
migration is completed.  By invoking wait_for_freed_hugetlb_folios()
before calling PageBuddy(), we ensure that PageBuddy() will succeed.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: c77c0a8 ("mm/hugetlb: defer freeing of huge pages if in non-task context")
Signed-off-by: Ge Yang <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Ge Yang

include/linux/hugetlb.h

@@ -682,6 +682,7 @@ struct huge_bootmem_page {
 
 int isolate_or_dissolve_huge_page(struct page *page, struct list_head *list);
 int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn);
+void wait_for_freed_hugetlb_folios(void);
 struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 				unsigned long addr, bool cow_from_owner);
 struct folio *alloc_hugetlb_folio_nodemask(struct hstate *h, int preferred_nid,
@@ -1066,6 +1067,10 @@ static inline int replace_free_hugepage_folios(unsigned long start_pfn,
 	return 0;
 }
 
+static inline void wait_for_freed_hugetlb_folios(void)
+{
+}
+
 static inline struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 					   unsigned long addr,
 					   bool cow_from_owner)

mm/hugetlb.c

@@ -2943,6 +2943,14 @@ int replace_free_hugepage_folios(unsigned long start_pfn, unsigned long end_pfn)
 	return ret;
 }
 
+void wait_for_freed_hugetlb_folios(void)
+{
+	if (llist_empty(&hpage_freelist))
+		return;
+
+	flush_work(&free_hpage_work);
+}
+
 typedef enum {
 	/*
 	 * For either 0/1: we checked the per-vma resv map, and one resv

mm/page_isolation.c

@@ -607,6 +607,16 @@ int test_pages_isolated(unsigned long start_pfn, unsigned long end_pfn,
 	struct zone *zone;
 	int ret;
 
+	/*
+	 * Due to the deferred freeing of hugetlb folios, the hugepage folios may
+	 * not immediately release to the buddy system. This can cause PageBuddy()
+	 * to fail in __test_page_isolated_in_pageblock(). To ensure that the
+	 * hugetlb folios are properly released back to the buddy system, we
+	 * invoke the wait_for_freed_hugetlb_folios() function to wait for the
+	 * release to complete.
+	 */
+	wait_for_freed_hugetlb_folios();
+
 	/*
 	 * Note: pageblock_nr_pages != MAX_PAGE_ORDER. Then, chunks of free
 	 * pages are not aligned to pageblock_nr_pages.