The patch titled
Subject: hugetlb: simplify prep_compound_gigantic_page ref count racing code
has been added to the -mm tree. Its filename is
hugetlb-simplify-prep_compound_gigantic_page-ref-count-racing-code.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/hugetlb-simplify-prep_compound_gigantic_page-ref-count-racing-code.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/hugetlb-simplify-prep_compound_gigantic_page-ref-count-racing-code.patch
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------------------------------------------------------
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");
}
/*
_
Patches currently in -mm which might be from mike.kravetz@xxxxxxxxxx are
hugetlb-simplify-prep_compound_gigantic_page-ref-count-racing-code.patch
hugetlb-drop-ref-count-earlier-after-page-allocation.patch
hugetlb-before-freeing-hugetlb-page-set-dtor-to-appropriate-value.patch
