[PATCH v2 4/4] mm: prohibit NULL deference exposed for unsupported non-blockable __GFP_NOFAIL

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

From: Barry Song <v-songbaohua@oppo.com>

When users allocate memory with the __GFP_NOFAIL flag, they might
incorrectly use it alongside GFP_ATOMIC, GFP_NOWAIT, etc. This kind
of non-blockable __GFP_NOFAIL is not supported and is pointless. If
we attempt and still fail to allocate memory for these users, we have
two choices:

    1. We could busy-loop and hope that some other direct reclamation or
    kswapd rescues the current process. However, this is unreliable
    and could ultimately lead to hard or soft lockups, which might not
    be well supported by some architectures.

    2. We could use BUG_ON to trigger a reliable system crash, avoiding
    exposing NULL dereference.

This patch chooses the second option because the first is unreliable. Even
if the process incorrectly using __GFP_NOFAIL is sometimes rescued, the
long latency might be unacceptable, especially considering that misusing
GFP_ATOMIC and __GFP_NOFAIL is likely to occur in atomic contexts with
strict timing requirements.

Cc: Michal Hocko <mhocko@suse.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/page_alloc.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cc179c3e68df..ed1bd8f595bd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4439,11 +4439,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 */
 	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
+		 * All existing users of the __GFP_NOFAIL are blockable
+		 * otherwise we introduce a busy loop with inside the page
+		 * allocator from non-sleepable contexts
 		 */
-		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
-			goto fail;
+		BUG_ON(!can_direct_reclaim);
 
 		/*
 		 * PF_MEMALLOC request from this context is rather bizarre
@@ -4474,7 +4474,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		cond_resched();
 		goto retry;
 	}
-fail:
+
 	warn_alloc(gfp_mask, ac->nodemask,
 			"page allocation failure: order:%u", order);
 got_pg:
-- 
2.34.1