Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 14, 2020 at 01:41:40PM -0700, Paul E. McKenney wrote:
> > And that enforces the GFP_NOLOCK allocation mode or some other solution
> > unless you make a new rule that calling call_rcu() is forbidden while
> > holding zone lock or any other lock which might be nested inside the
> > GFP_NOWAIT zone::lock held region.
> 
> Again, you are correct.  Maybe the forecasted weekend heat will cause
> my brain to hallucinate a better solution, but in the meantime, the
> GFP_NOLOCK approach looks good from this end.

So I hate __GFP_NO_LOCKS for a whole number of reasons:

 - it should be called __GFP_LOCKLESS if anything
 - it sprinkles a bunch of ugly branches around the allocator fast path
 - it only works for order==0

Combined I really odn't think this should be a GFP flag. How about a
special purpose allocation function, something like so..

---
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 901a21f61d68..cdec9c99fba7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4875,6 +4875,47 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
 }
 EXPORT_SYMBOL(__alloc_pages_nodemask);
 
+struct page *__rmqueue_lockless(struct zone *zone, struct per_cpu_pages *pcp)
+{
+	struct list_head *list;
+	struct page *page;
+	int migratetype;
+
+	for (migratetype = 0; migratetype < MIGRATE_PCPTYPES; migratetype++) {
+		list = &pcp->list[migratetype];
+		page = list_first_entry_or_null(list, struct page, lru);
+		if (page && check_new_pcp(page)) {
+			list_del(&page->lru);
+			pcp->count--;
+			return page;
+		}
+	}
+
+	return NULL;
+}
+
+struct page *__alloc_page_lockless(void)
+{
+	struct zonelist *zonelist = node_zonelist(numa_node_id(), GFP_KERNEL);
+	struct per_cpu_pages *pcp;
+	struct page *page = NULL;
+	unsigned long flags;
+	struct zoneref *z;
+	struct zone *zone;
+
+	for_each_zone_zonelist(zone, z, zonelist, ZONE_NORMAL) {
+		local_irq_save(flags);
+		pcp = &this_cpu_ptr(zone->pageset)->pcp;
+		page = __rmqueue_lockless(zone, pcp);
+		local_irq_restore(flags);
+
+		if (page)
+			break;
+	}
+
+	return page;
+}
+
 /*
  * Common helper functions. Never use with __GFP_HIGHMEM because the returned
  * address cannot represent highmem pages. Use alloc_pages and then kmap if