Re: [PATCH v4 3/3] mm: warn about illegal __GFP_NOFAIL usage in a more appropriate location and manner

[David Hildenbrand <david@redhat.com>]

On 30.08.24 22:28, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Three points for this change:
> 
> 1. We should consolidate all warnings in one place. Currently, the
>     order > 1 warning is in the hotpath, while others are in less
>     likely scenarios. Moving all warnings to the slowpath will reduce
>     the overhead for order > 1 and increase the visibility of other
>     warnings.
> 
> 2. We currently have two warnings for order: one for order > 1 in
>     the hotpath and another for order > costly_order in the laziest
>     path. I suggest standardizing on order > 1 since it’s been in
>     use for a long time.
> 
> 3. We don't need to check for __GFP_NOWARN in this case. __GFP_NOWARN
>     is meant to suppress allocation failure reports, but here we're
>     dealing with bug detection, not allocation failures. So replace
>     WARN_ON_ONCE_GFP by WARN_ON_ONCE.
> 
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>   mm/page_alloc.c | 50 ++++++++++++++++++++++++-------------------------
>   1 file changed, 25 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c81ee5662cc7..e790b4227322 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
>   {
>   	struct page *page;
>   
> -	/*
> -	 * We most definitely don't want callers attempting to
> -	 * allocate greater than order-1 page units with __GFP_NOFAIL.
> -	 */
> -	WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
> -
>   	if (likely(pcp_allowed_order(order))) {
>   		page = rmqueue_pcplist(preferred_zone, zone, order,
>   				       migratetype, alloc_flags);
> @@ -4175,6 +4169,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>   {
>   	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
>   	bool can_compact = gfp_compaction_allowed(gfp_mask);
> +	bool nofail = gfp_mask & __GFP_NOFAIL;
>   	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
>   	struct page *page = NULL;
>   	unsigned int alloc_flags;
> @@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>   	unsigned int zonelist_iter_cookie;
>   	int reserve_flags;
>   
> +	if (unlikely(nofail)) {
> +		/*
> +		 * We most definitely don't want callers attempting to
> +		 * allocate greater than order-1 page units with __GFP_NOFAIL.
> +		 */

Acked-by: David Hildenbrand <david@redhat.com>

Should we also clarify that in the docs? Currently we have "Using this 
flag for costly allocations is _highly_ discouraged."

We'd likely want to say something like "Allocating pages from the buddy 
with __GFP_NOFAIL and order > 1 is not supported."

-- 
Cheers,

David / dhildenb