On 1/25/21 7:10 PM, Muchun Song wrote:
> The VM_BUG_ON_PAGE avoids the generation of any code, even if that
> expression has side-effects when !CONFIG_DEBUG_VM.
>
> Fixes: e5dfacebe4a4 ("mm/hugetlb.c: just use put_page_testzero() instead of page_count()")
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
> mm/hugetlb.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
Thanks for finding and fixing this! My bad for not noticing when the bug
was introduced.
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a6bad1f686c5..082ed643020b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2047,13 +2047,16 @@ static int gather_surplus_pages(struct hstate *h, long delta)
>
> /* Free the needed pages to the hugetlb pool */
> list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
> + int zeroed;
> +
> if ((--needed) < 0)
> break;
> /*
> * This page is now managed by the hugetlb allocator and has
> * no users -- drop the buddy allocator's reference.
> */
> - VM_BUG_ON_PAGE(!put_page_testzero(page), page);
> + zeroed = put_page_testzero(page);
> + VM_BUG_ON_PAGE(!zeroed, page);
> enqueue_huge_page(h, page);
I was wondering why this was not causing any problems. We are putting the
hugetlb page on the free list with a count of 1. There is no check in the
enqueue code. When we dequeue the page, set_page_refcounted() is used to
set the count to 1 without looking at the current value. And, all the other
VM_DEBUG macros are off so we mostly do not notice the bug.
Thanks again,
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
--
Mike Kravetz
> }
> free:
>
