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

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On Wed, 24 Jul 2024, Barry Song wrote:\n
From: Barry Song <v-songbaohua@xxxxxxxx>

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@xxxxxxxxxxxx>




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