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

[Chen Ridong]

On 2026/5/7 18:54, Chen Wandun wrote:
> 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@xxxxxxxxxxx>
> ---
>  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);

Make sense.

BTW, how did you find this issue?

Reviewed-by: Chen Ridong <chenridong@xxxxxxxxxxxxxxx>

-- 
Best regards,
Ridong