[RFC v1 5/5] maple_tree: replace preallocation with slub percpu array prefill

[Vlastimil Babka <vbabka@suse.cz>]

With the percpu array we can try not doing the preallocations in maple
tree, and instead make sure the percpu array is prefilled, and using
GFP_ATOMIC in places that relied on the preallocation (in case we miss
or fail trylock on the array), i.e. mas_store_prealloc(). For now simply
add __GFP_NOFAIL there as well.

First I tried to change mas_node_count_gfp() to not preallocate anything
anywhere, but that lead to warns and panics, even though the other
caller mas_node_count() uses GFP_NOWAIT | __GFP_NOWARN so it has no
guarantees... So I changed just mas_preallocate(). I let it still to
truly preallocate a single node, but maybe it's not necessary?
---
 lib/maple_tree.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 7a8e7c467d7c..5a209d88c318 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -5534,7 +5534,12 @@ void mas_store_prealloc(struct ma_state *mas, void *entry)
 
 	mas_wr_store_setup(&wr_mas);
 	trace_ma_write(__func__, mas, 0, entry);
+
+retry:
 	mas_wr_store_entry(&wr_mas);
+	if (unlikely(mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)))
+		goto retry;
+
 	MAS_WR_BUG_ON(&wr_mas, mas_is_err(mas));
 	mas_destroy(mas);
 }
@@ -5550,9 +5555,10 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc);
 int mas_preallocate(struct ma_state *mas, gfp_t gfp)
 {
 	int ret;
+	int count = 1 + mas_mt_height(mas) * 3;
 
-	mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp);
-	mas->mas_flags |= MA_STATE_PREALLOC;
+	mas_node_count_gfp(mas, 1, gfp);
+	kmem_cache_prefill_percpu_array(maple_node_cache, count, gfp);
 	if (likely(!mas_is_err(mas)))
 		return 0;
 
-- 
2.41.0