Re: [PATCH v3 0/4] mm: clarify nofail memory allocation

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

> > > Patch 4/4: We will move the order > 1 check from the current fast path
> > > to the slow path and extend
> > >                  the check of gfp_direct_reclaim flag also in the slow path.
> >
> > OK, let's have that go in now as well.

Hi Michal and Vlastimil,
Could you please review the changes below before I send v4 for patch 4/4?

1. We should consolidate all warnings in one place. Currently, the order > 1 warning is
in the hotpath, while others are in less likely scenarios. Moving all warnings to the
slowpath will reduce the overhead for order > 1 and increase the visibility of other
warnings.

2. We currently have two warnings for order: one for order > 1 in the hotpath and another
for order > costly_order in the laziest path. I suggest standardizing on order > 1 since
it’s been in use for a long time.

3.I don't think we need to check for __GFP_NOWARN in this case. __GFP_NOWARN is
meant to suppress allocation failure reports, but here we're dealing with bug detection, not
allocation failures.
So I'd rather use WARN_ON_ONCE than WARN_ON_ONCE_GFP.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c81ee5662cc7..0d3dd679d0ab 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3033,12 +3033,6 @@ struct page *rmqueue(struct zone *preferred_zone,
 {
 	struct page *page;
 
-	/*
-	 * We most definitely don't want callers attempting to
-	 * allocate greater than order-1 page units with __GFP_NOFAIL.
-	 */
-	WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
-
 	if (likely(pcp_allowed_order(order))) {
 		page = rmqueue_pcplist(preferred_zone, zone, order,
 				       migratetype, alloc_flags);
@@ -4174,6 +4168,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
 {
 	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
+	bool nofail = gfp_mask & __GFP_DIRECT_RECLAIM;
 	bool can_compact = gfp_compaction_allowed(gfp_mask);
 	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
 	struct page *page = NULL;
@@ -4187,6 +4182,25 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	unsigned int zonelist_iter_cookie;
 	int reserve_flags;
 
+	if (nofail) {
+		/*
+		 * We most definitely don't want callers attempting to
+		 * allocate greater than order-1 page units with __GFP_NOFAIL.
+		 */
+		WARN_ON_ONCE(order > 1);
+		/*
+		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
+		 * otherwise, we may result in lockup.
+		 */
+		WARN_ON_ONCE(!can_direct_reclaim);
+		/*
+		 * PF_MEMALLOC request from this context is rather bizarre
+		 * because we cannot reclaim anything and only can loop waiting
+		 * for somebody to do a work for us.
+		 */
+		WARN_ON_ONCE(current->flags & PF_MEMALLOC);
+	}
+
 restart:
 	compaction_retries = 0;
 	no_progress_loops = 0;
@@ -4404,29 +4418,15 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 * Make sure that __GFP_NOFAIL request doesn't leak out and make sure
 	 * we always retry
 	 */
-	if (gfp_mask & __GFP_NOFAIL) {
+	if (nofail) {
 		/*
-		 * All existing users of the __GFP_NOFAIL are blockable, so warn
-		 * of any new users that actually require GFP_NOWAIT
+		 * Lacking direct_reclaim we can't do anything to reclaim memory,
+		 * we disregard these unreasonable nofail requests and still
+		 * return NULL
 		 */
-		if (WARN_ON_ONCE_GFP(!can_direct_reclaim, gfp_mask))
+		if (!can_direct_reclaim)
 			goto fail;
 
-		/*
-		 * PF_MEMALLOC request from this context is rather bizarre
-		 * because we cannot reclaim anything and only can loop waiting
-		 * for somebody to do a work for us
-		 */
-		WARN_ON_ONCE_GFP(current->flags & PF_MEMALLOC, gfp_mask);
-
-		/*
-		 * non failing costly orders are a hard requirement which we
-		 * are not prepared for much so let's warn about these users
-		 * so that we can identify them and convert them to something
-		 * else.
-		 */
-		WARN_ON_ONCE_GFP(costly_order, gfp_mask);
-
 		/*
 		 * Help non-failing allocations by giving some access to memory
 		 * reserves normally used for high priority non-blocking

> >
> > --
> > Michal Hocko
> > SUSE Labs

Thanks
Barry