From: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Subject: hugetlb: simplify prep_compound_gigantic_page ref count racing code Code in prep_compound_gigantic_page waits for a rcu grace period if it notices a temporarily inflated ref count on a tail page. This was due to the identified potential race with speculative page cache references which could only last for a rcu grace period. This is overly complicated as this situation is VERY unlikely to ever happen. Instead, just quickly return an error. Also, only print a warning in prep_compound_gigantic_page instead of multiple callers. Link: https://lkml.kernel.org/r/20210809184832.18342-2-mike.kravetz@xxxxxxxxxx Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Mina Almasry <almasrymina@xxxxxxxxxx> Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx> Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxxxx> Cc: Oscar Salvador <osalvador@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/hugetlb.c | 17 +++++++---------- 1 file changed, 7 insertions(+), 10 deletions(-) --- a/mm/hugetlb.c~hugetlb-simplify-prep_compound_gigantic_page-ref-count-racing-code +++ a/mm/hugetlb.c @@ -1657,16 +1657,14 @@ static bool prep_compound_gigantic_page( * cache adding could take a ref on a 'to be' tail page. * We need to respect any increased ref count, and only set * the ref count to zero if count is currently 1. If count - * is not 1, we call synchronize_rcu in the hope that a rcu - * grace period will cause ref count to drop and then retry. - * If count is still inflated on retry we return an error and - * must discard the pages. + * is not 1, we return an error. An error return indicates + * the set of pages can not be converted to a gigantic page. + * The caller who allocated the pages should then discard the + * pages using the appropriate free interface. */ if (!page_ref_freeze(p, 1)) { - pr_info("HugeTLB unexpected inflated ref count on freshly allocated page\n"); - synchronize_rcu(); - if (!page_ref_freeze(p, 1)) - goto out_error; + pr_warn("HugeTLB page can not be used due to unexpected inflated ref count\n"); + goto out_error; } set_page_count(p, 0); set_compound_head(p, page); @@ -1830,7 +1828,6 @@ retry: retry = true; goto retry; } - pr_warn("HugeTLB page can not be used due to unexpected inflated ref count\n"); return NULL; } } @@ -2828,8 +2825,8 @@ static void __init gather_bootmem_preall prep_new_huge_page(h, page, page_to_nid(page)); put_page(page); /* add to the hugepage allocator */ } else { + /* VERY unlikely inflated ref count on a tail page */ free_gigantic_page(page, huge_page_order(h)); - pr_warn("HugeTLB page can not be used due to unexpected inflated ref count\n"); } /* _
