Re: [PATCH] mm: mempolicy: fix automatic numa balancing for shmem

[Johannes Weiner]

On Wed, Jul 01, 2026 at 11:33:38AM -0400, Gregory Price wrote:
> Something i found while seeing if i could make ZONE_NORMAL nodes more
> reliably hotpluggable:
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index dd2717ff94bf..9ceeb56574ef 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -682,7 +682,15 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
>  	 * we won't be able to do that under the lock.
>  	 */
>  	if (unlikely(can_alloc && !READ_ONCE(new_pool))) {
> -		page = alloc_pages(gfp_nested_mask(alloc_flags),
> +		/*
> +		 * The stack depot pool is a global, never-freed allocation.
> +		 * Use alloc_pages_node() on the CPU-local node instead of
> +		 * alloc_pages() so the pool does not inherit a transient task's
> +		 * NUMA mempolicy (e.g. MPOL_BIND to a CPU-less/bound node), which
> +		 * would strand this long-lived page on that node forever.
> +		 */
> +		page = alloc_pages_node(numa_node_id(),
> +				   gfp_nested_mask(alloc_flags),
>  				   DEPOT_POOL_ORDER);
>  		if (page)
>  			prealloc = page_address(page);
> 
> This is a global, permanently allocated, resource that inherits a task
> mempolicy's placement because that task *happened* to be the first one
> to touch it.
> 
> There are many alloc_pages() calls (155 instances kernel-wide) that
> inherit a task mempolicy when that's probably not what we want.
> 
> alloc_pages() is called in: net/, lib/, kexec_core/, drivers/, arch/
> 
> you can imagine a task setting `set_mempolicy(INTERLEAVE, ALL)` and the
> result is a bunch of random driver memory gets spread all over the place
> along with the task's heap.  Is that really what the caller wanted, or
> did they just want userland data spread about?
> 
> But at this point it's a 20 year old interface, not much we can do about
> it without making *someone* sad :[
> 
> I considered proposing MPOL_F_MOVABLE_ONLY to mean (roughly) "userland
> memory only" - and then slowly trying to migrate numactl to make this
> the default.

Hm. Kernel allocations that are totally incidental like the stackdepot
example above should not follow task policy. But there are kernel
allocations (kernel stack, inodes, pipes) that are directly allocated
on behalf of a process, and so probably SHOULD follow task policy.

That's an annotation problem that I think we have solved already,
because cgroups need the same distinction for what allocations to
charge to the current task's cgroup context.

We could rename __GFP_ACCOUNT / SLAB_ACCOUNT to __GFP_TASKPOLICY /
SLAB_TASKPOLICY or something, and have mempolicy follow it too.

There is still the whole "changing 20 year old behavior" aspect, but I
think the polarity works in our favor: big important allocations have
already been following the policy correctly. The behavior changes
primarily for smaller, random allocations.