Re: [PATCH v4 2/3] mm: document __GFP_NOFAIL must be blockable

[David Hildenbrand <david@redhat.com>]

On 30.08.24 22:28, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Non-blocking allocation with __GFP_NOFAIL is not supported and may still
> result in NULL pointers (if we don't return NULL, we result in busy-loop
> within non-sleepable contexts):
> 
> static inline struct page *
> __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> 						struct alloc_context *ac)
> {
> 	...
> 	/*
> 	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
> 	 * we always retry
> 	 */
> 	if (gfp_mask & __GFP_NOFAIL) {
> 		/*
> 		 * All existing users of the __GFP_NOFAIL are blockable, so warn
> 		 * of any new users that actually require GFP_NOWAIT
> 		 */
> 		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
> 			goto fail;
> 		...
> 	}
> 	...
> fail:
> 	warn_alloc(gfp_mask, ac->nodemask,
> 			"page allocation failure: order:%u", order);
> got_pg:
> 	return page;
> }
> 
> Highlight this in the documentation of __GFP_NOFAIL so that non-mm
> subsystems can reject any illegal usage of __GFP_NOFAIL with GFP_ATOMIC,
> GFP_NOWAIT, etc.
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: Davidlohr Bueso <dave@stgolabs.net>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: "Eugenio Pérez" <eperezma@redhat.com>
> Cc: Hailong.Liu <hailong.liu@oppo.com>
> Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Cc: Jason Wang <jasowang@redhat.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Kees Cook <kees@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---

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

-- 
Cheers,

David / dhildenb