[PATCH] cgroup/cpuset: move PF_EXITING check before __GFP_HARDWALL in cpuset_current_node_allowed()

[Chen Wandun <chenwandun1@gmail.com>]

Since prepare_alloc_pages() unconditionally adds __GFP_HARDWALL for the
fast path when cpusets are enabled, the __GFP_HARDWALL check in
cpuset_current_node_allowed() causes the PF_EXITING escape path to be
skipped on the first allocation attempt.  This makes it unreachable in
the common case, so dying tasks can get stuck in direct reclaim or even
trigger OOM while trying to exit, despite being allowed to allocate from
any node.

Move the PF_EXITING check before __GFP_HARDWALL so that dying tasks
can allocate memory from any node to exit quickly, even when cpusets
are enabled.

Also update the function comment to reflect the actual behavior of
prepare_alloc_pages() and the corrected check ordering.

Signed-off-by: Chen Wandun <chenwandun@lixiang.com>
---
 kernel/cgroup/cpuset.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e3a081a07c6d..a48901a0416a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4176,11 +4176,11 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs)
  * current's mems_allowed, yes.  If it's not a __GFP_HARDWALL request and this
  * node is set in the nearest hardwalled cpuset ancestor to current's cpuset,
  * yes.  If current has access to memory reserves as an oom victim, yes.
- * Otherwise, no.
+ * If the current task is PF_EXITING, yes. Otherwise, no.
  *
  * GFP_USER allocations are marked with the __GFP_HARDWALL bit,
  * and do not allow allocations outside the current tasks cpuset
- * unless the task has been OOM killed.
+ * unless the task has been OOM killed or is exiting.
  * GFP_KERNEL allocations are not so marked, so can escape to the
  * nearest enclosing hardwalled ancestor cpuset.
  *
@@ -4194,7 +4194,9 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs)
  * The first call here from mm/page_alloc:get_page_from_freelist()
  * has __GFP_HARDWALL set in gfp_mask, enforcing hardwall cpusets,
  * so no allocation on a node outside the cpuset is allowed (unless
- * in interrupt, of course).
+ * in interrupt, of course).  The PF_EXITING check must therefore
+ * come before the __GFP_HARDWALL check, otherwise a dying task
+ * would be blocked on the fast path.
  *
  * The second pass through get_page_from_freelist() doesn't even call
  * here for GFP_ATOMIC calls.  For those calls, the __alloc_pages()
@@ -4204,6 +4206,7 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs)
  *	in_interrupt - any node ok (current task context irrelevant)
  *	GFP_ATOMIC   - any node ok
  *	tsk_is_oom_victim   - any node ok
+ *	PF_EXITING   - any node ok (let dying task exit quickly)
  *	GFP_KERNEL   - any node in enclosing hardwalled cpuset ok
  *	GFP_USER     - only nodes in current tasks mems allowed ok.
  */
@@ -4223,11 +4226,10 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask)
 	 */
 	if (unlikely(tsk_is_oom_victim(current)))
 		return true;
-	if (gfp_mask & __GFP_HARDWALL)	/* If hardwall request, stop here */
-		return false;
-
 	if (current->flags & PF_EXITING) /* Let dying task have memory */
 		return true;
+	if (gfp_mask & __GFP_HARDWALL)	/* If hardwall request, stop here */
+		return false;
 
 	/* Not hardwall and node outside mems_allowed: scan up cpusets */
 	spin_lock_irqsave(&callback_lock, flags);
-- 
2.43.0