On Sat, Jul 10, 2021 at 8:25 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> 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.
Right. The race is very very small. IMHO, that does not complicate
the code is the right thing to do.
>
> 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 | 15 +++++----------
> 1 file changed, 5 insertions(+), 10 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 924553aa8f78..e59ebba63da7 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1657,16 +1657,12 @@ 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 and caller must discard the
> + * pages.
Shall we add more details about why we discard the pages?
Thanks.
> */
> 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 +1826,6 @@ 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 +2823,8 @@ 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
>
