linux-kernel - [PATCH 1/3] mm/vmalloc: simplify merge_or_add_vmap

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Date:   Wed, 27 May 2020 22:50:52 +0200
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH 1/3] mm/vmalloc: simplify merge_or_add_vmap_area() func.

Currently when a VA is deallocated and is about to be
placed back to the tree, it can be either: merged with
next/prev neighbors or inserted if not coalesced.

On those steps the tree can be populated several times.
For example when both neighbors are merged. It can be
avoided and simplified in fact.

Therefore do it only once when VA points to final merged
area, after all manipulations: merging/removing/inserting.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 410bfe26ee73..a875eef1e2c2 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -753,9 +753,6 @@ merge_or_add_vmap_area(struct vmap_area *va,
 		if (sibling->va_start == va->va_end) {
 			sibling->va_start = va->va_start;
 
-			/* Check and update the tree if needed. */
-			augment_tree_propagate_from(sibling);
-
 			/* Free vmap_area object. */
 			kmem_cache_free(vmap_area_cachep, va);
 
@@ -775,14 +772,18 @@ merge_or_add_vmap_area(struct vmap_area *va,
 	if (next->prev != head) {
 		sibling = list_entry(next->prev, struct vmap_area, list);
 		if (sibling->va_end == va->va_start) {
-			sibling->va_end = va->va_end;
-
-			/* Check and update the tree if needed. */
-			augment_tree_propagate_from(sibling);
-
+			/*
+			 * If both neighbors are coalesced, it is important
+			 * to unlink the "next" node first, followed by merging
+			 * with "previous" one. Otherwise the tree might not be
+			 * fully populated if a sibling's augmented value is
+			 * "normalized" because of rotation operations.
+			 */
 			if (merged)
 				unlink_va(va, root);
 
+			sibling->va_end = va->va_end;
+
 			/* Free vmap_area object. */
 			kmem_cache_free(vmap_area_cachep, va);
 
@@ -793,11 +794,13 @@ merge_or_add_vmap_area(struct vmap_area *va,
 	}
 
 insert:
-	if (!merged) {
+	if (!merged)
 		link_va(va, root, parent, link, head);
-		augment_tree_propagate_from(va);
-	}
 
+	/*
+	 * Last step is to check and update the tree.
+	 */
+	augment_tree_propagate_from(va);
 	return va;
 }
 
-- 
2.20.1