Re: [PATCH 2/5] mm: Document __GFP_NOFAIL must be blockable

[Davidlohr Bueso <dave@stgolabs.net>]

On Wed, 24 Jul 2024, Barry Song wrote:\n
>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.
>

Acked-by: Davidlohr Bueso <dave@stgolabs.net>