6.6-stable review patch. If anyone has any objections, please let me know.
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From: Ilias Stamatis ilstam@amazon.com
[ Upstream commit 6fb3acdebf65a72df0a95f9fd2c901ff2bc9a3a2 ]
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 performance improvements in systems where /proc/iomem is ~5k lines long. Calling mmap() on /dev/mem in such platforms takes 700-1500μs without the optimisation and 10-50μs with the optimisation.
Note that even though commit 97523a4edb7b removed the 'sibling_only' parameter from next_resource(), newer kernels have basically reinstated it under the name 'skip_children'.
Link: https://lore.kernel.org/all/20251124165349.3377826-1-ilstam@amazon.com/T/#u Fixes: 97523a4edb7b ("kernel/resource: remove first_lvl / siblings_only logic") Signed-off-by: Ilias Stamatis ilstam@amazon.com Acked-by: David Hildenbrand (Red Hat) david@kernel.org Cc: Andriy Shevchenko andriy.shevchenko@linux.intel.com Cc: Baoquan He bhe@redhat.com Cc: "Huang, Ying" huang.ying.caritas@gmail.com Cc: Nadav Amit nadav.amit@gmail.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/resource.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/kernel/resource.c b/kernel/resource.c index 67410132b40e1..dca8581eb3992 100644 --- a/kernel/resource.c +++ b/kernel/resource.c @@ -336,6 +336,8 @@ 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) @@ -346,7 +348,7 @@ 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; @@ -357,6 +359,12 @@ 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;