linux-kernel - [PATCH] Reinstate "resource: avoid unnecessary lookups in find_next_iomem

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Message-ID: <20251124165349.3377826-1-ilstam@amazon.com>
Date: Mon, 24 Nov 2025 16:53:49 +0000
From: Ilias Stamatis <ilstam@...zon.com>
To: <akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>
CC: <linux-mm@...ck.org>, <david@...nel.org>, <nadav.amit@...il.com>,
	<huang.ying.caritas@...il.com>, <andriy.shevchenko@...ux.intel.com>,
	<bhe@...hat.com>, <nh-open-source@...zon.com>, Ilias Stamatis
	<ilstam@...zon.com>
Subject: [PATCH] Reinstate "resource: avoid unnecessary lookups in find_next_iomem_res()"

Commit 97523a4edb7b ("kernel/resource: remove first_lvl / siblings_only
logic") removed an optimization introduced by commit 756398750e11
("resource: avoid unnecessary lookups in find_next_iomem_res()"). That
was not called out in the message of the first commit explicitly so it's
not entirely clear whether removing the optimization happened
inadvertently or not.

As the original commit message of the optimization explains there is no
point considering the children of a subtree in find_next_iomem_res() if
the top level range does not match. Reinstating the optimization results
in significant performance improvements in systems with very large iomem
maps when mmaping /dev/mem.

Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
---
 kernel/resource.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/resource.c b/kernel/resource.c
index b9fa2a4ce089..e4e9bac12e6e 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -341,6 +341,8 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 			       unsigned long flags, unsigned long desc,
 			       struct resource *res)
 {
+	/* Skip children until we find a top level range that matches */
+	bool skip_children = true;
 	struct resource *p;
 
 	if (!res)
@@ -351,7 +353,7 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 
 	read_lock(&resource_lock);
 
-	for_each_resource(&iomem_resource, p, false) {
+	for_each_resource(&iomem_resource, p, skip_children) {
 		/* If we passed the resource we are looking for, stop */
 		if (p->start > end) {
 			p = NULL;
@@ -362,6 +364,12 @@ static int find_next_iomem_res(resource_size_t start, resource_size_t end,
 		if (p->end < start)
 			continue;
 
+		/*
+		 * We found a top level range that matches what we are looking
+		 * for. Time to start checking children too.
+		 */
+		skip_children = false;
+
 		/* Found a match, break */
 		if (is_type_match(p, flags, desc))
 			break;
-- 
2.43.0