[PATCH v2 1/3] hugetlb: simplify prep_compound_gigantic_page ref count racing code

[Mike Kravetz]

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.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dfc940d5221d..791ee699d635 100644
--- 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 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
 				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");
 		}
 
 		/*
-- 
2.31.1