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@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8ea35ba..3658398 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1657,16 +1657,14 @@ static bool prep_compound_gigantic_page(struct page *page, unsigned int order)
                 * 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_prealloc(void)
                        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");
                }
 
                /*