mm: document __GFP_NOFAIL must be blockable

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: "Eugenio Pérez" <[email protected]>
Cc: Hailong.Liu <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: Jason Wang <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Maxime Coquelin <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Cc: Xuan Zhuo <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Xie Yongji <[email protected]>
Cc: Yafang Shao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions