[PATCH v2 2/4] mm: Document __GFP_NOFAIL must be blockable

[Barry Song <21cnbao@gmail.com>]

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: Michal Hocko <mhocko@suse.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 include/linux/gfp_types.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 313be4ad79fd..4a1fa7706b0c 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -215,7 +215,8 @@ enum {
  * the caller still has to check for failures) while costly requests try to be
  * not disruptive and back off even without invoking the OOM killer.
  * The following three modifiers might be used to override some of these
- * implicit rules.
+ * implicit rules. Please note that all of them must be used along with
+ * %__GFP_DIRECT_RECLAIM flag.
  *
  * %__GFP_NORETRY: The VM implementation will try only very lightweight
  * memory direct reclaim to get some memory under memory pressure (thus
@@ -246,6 +247,8 @@ enum {
  * cannot handle allocation failures. The allocation could block
  * indefinitely but will never return with failure. Testing for
  * failure is pointless.
+ * It _must_ be blockable and used together with __GFP_DIRECT_RECLAIM.
+ * It should _never_ be used in non-sleepable contexts.
  * New users should be evaluated carefully (and the flag should be
  * used only when there is no reasonable failure policy) but it is
  * definitely preferable to use the flag rather than opencode endless
-- 
2.34.1