[PATCH] cgroup/cpuset: skip hardwall ancestor scan in v2 mode in cpuset_current_node_allowed()

[Chen Wandun]

In cgroup v2, the non-hardwall fallthrough path in
cpuset_current_node_allowed() always ends up allowing the allocation:

  - CS_MEM_EXCLUSIVE and CS_MEM_HARDWALL are v1-only flags, toggled
    only via the cpuset.mem_exclusive / cpuset.mem_hardwall files
    which do not exist in v2.  Neither flag is ever set on any cpuset
    (including top_cpuset) in pure v2 mode.
  - As a result, nearest_hardwall_ancestor() always walks up to
    top_cpuset.
  - top_cpuset.mems_allowed is set to node_possible_map in v2 mode,
    so node_isset() on it is always true for any valid node.

The whole scan therefore boils down to taking callback_lock, walking
to the root and returning true.  Short-circuit it by returning true
directly when is_in_v2_mode() holds, sparing the callback_lock
acquisition and the pointless walk.

Place the short-circuit after the __GFP_HARDWALL check so that the
generic hardwall enforcement for GFP_USER allocations remains in
effect: __GFP_HARDWALL requests still return false when the node is
outside mems_allowed, preserving cpuset.mems constraints for
__alloc_pages() callers (which prepare_alloc_pages() marks
__GFP_HARDWALL unconditionally when cpusets are enabled).

Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Chen Wandun <chenwandun@xxxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a48901a0416a..b539f5b4d21e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4231,6 +4231,9 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	if (gfp_mask & __GFP_HARDWALL)	/* If hardwall request, stop here */
 		return false;
 
+	if (is_in_v2_mode())
+		return true;
+
 	/* Not hardwall and node outside mems_allowed: scan up cpusets */
 	spin_lock_irqsave(&callback_lock, flags);
 
-- 
2.43.0