diff mbox series

mm: hugetlb: fix missing put_page in gather_surplus_pages()

Message ID 20210126031009.96266-1-songmuchun@bytedance.com (mailing list archive)
State New
Headers show
Series mm: hugetlb: fix missing put_page in gather_surplus_pages() | expand

Commit Message

Muchun Song Jan. 26, 2021, 3:10 a.m. UTC
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@bytedance.com>
Cc: <stable@vger.kernel.org>
---
 mm/hugetlb.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Comments

Mike Kravetz Jan. 26, 2021, 4:31 a.m. UTC | #1
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@bytedance.com>
> Cc: <stable@vger.kernel.org>
> ---
>  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@oracle.com>
Muchun Song Jan. 26, 2021, 6:10 a.m. UTC | #2
On Tue, Jan 26, 2021 at 12:31 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> 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@bytedance.com>
> > Cc: <stable@vger.kernel.org>
> > ---
> >  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.

Yeah. You are right. I also thought about this question.
It is a very hidden bug.

>
> Thanks again,
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
>
> --
> Mike Kravetz
>
> >       }
> >  free:
> >
Miaohe Lin Jan. 26, 2021, 6:20 a.m. UTC | #3
Hi:
On 2021/1/26 11:10, 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@bytedance.com>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/hugetlb.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> 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);
>  	}
>  free:
> 

Good Catch! Thanks.

Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
diff mbox series

Patch

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);
 	}
 free: