Re: [PATCH 1/1] lib/alloc_tag: fix RCU imbalance in pgalloc_tag_get()

[Vlastimil Babka]

On 6/2/24 1:38 AM, Suren Baghdasaryan wrote:
> put_page_tag_ref() should be called only when get_page_tag_ref() returns
> a valid reference because only in that case get_page_tag_ref() enters RCU
> read section while put_page_tag_ref() will call rcu_read_unlock() even if
> the provided reference is NULL. Fix pgalloc_tag_get() which does not
> follow this rule causing RCU imbalance. Add a warning in put_page_tag_ref()
> to catch any future mistakes.
> 
> Fixes: cc92eba1c88b ("mm: fix non-compound multi-order memory accounting in __free_pages")
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-lkp/202405271029.6d2f9c4c-lkp@xxxxxxxxx
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  include/linux/pgalloc_tag.h | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
> index 86ba5d33e43b..9cacadbd61f8 100644
> --- a/include/linux/pgalloc_tag.h
> +++ b/include/linux/pgalloc_tag.h
> @@ -37,6 +37,9 @@ static inline union codetag_ref *get_page_tag_ref(struct page *page)
>  
>  static inline void put_page_tag_ref(union codetag_ref *ref)
>  {
> +	if (WARN_ON(!ref))
> +		return;
> +
>  	page_ext_put(page_ext_from_codetag_ref(ref));
>  }
>  
> @@ -102,9 +105,11 @@ static inline struct alloc_tag *pgalloc_tag_get(struct page *page)
>  		union codetag_ref *ref = get_page_tag_ref(page);
>  
>  		alloc_tag_sub_check(ref);
> -		if (ref && ref->ct)
> -			tag = ct_to_alloc_tag(ref->ct);
> -		put_page_tag_ref(ref);
> +		if (ref) {
> +			if (ref->ct)
> +				tag = ct_to_alloc_tag(ref->ct);
> +			put_page_tag_ref(ref);
> +		}
>  	}
>  
>  	return tag;
> 
> base-commit: 065d3634d60843b8e338d405b844cc7f2e5e1c66