Re: [PATCH] mm/damon: prevent migration fallback to non-target nodes

[Jiahui Zhang]

Hi SJ,

Thank you for the review.

On Wed, Jul 08, 2026 at 07:11:33AM -0700, SJ Park wrote:
> So this patch is introducing a small behavioral change.  May I ask
> why you think the new behavior is better?  Have you find some 
> negative user impacts from the current behavior?

Consider a two-node tiered system, where node 0 is the fast tier and 
node 1 is a CPU-less slow tier, such as CXL memory.  Suppose node 0 
is nearly full and the user wants to promote hot regions (which reside
on node 1) to node 0 with a command like:

  sudo damo start --ops vaddr --target_pid ${workload_pid} \
      --damos_action migrate_hot 0 \
      --damos_access_rate 70% max

Without the __GFP_THISNODE flag, when the memory allocator finds that
node 0 is nearly full, it can fall back to node 1 without waking up 
kswapd. Then the pages allocated for migrate_pages() are still on node 1, 
and the regions that are expected to be promoted to node 0 are only moved
 to different physical pages on node 1.

As a result, the NUMA node placement does not change, but the following
command still observes successful migrations:

  sudo bpftrace -e '
  tracepoint:migrate:mm_migrate_pages /args->reason == 9/
  {
      @succeeded_pages = sum(args->succeeded);
  }
  interval:s:1
  {
      time("%H:%M:%S ");
      print(@succeeded_pages);
      clear(@succeeded_pages);
  }'

I've also checked other page migration paths, such as alloc_demote_folio(),
 alloc_misplaced_dst_folio() and do_move_pages_to_node(), all of which set
the __GFP_THISNODE flag.

I am not sure whether the original design intended to provide only
best-effort semantics, but I found this behavior very confusing in my
experiment.

> We as the kernel community disallow [1] anonymous patch submissions.  Is zjh
> your preferred identity?  Also, it is not a strict rule, but we as the mm
> community prefer using a formal name if that's ok.

Sorry I did not notice this.  If you think the GFP flag should be added, I will
send a v2 of the patch and correct the Signed-off-by issue.

Thanks,
Jiahui Zhang