Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Gregory Price <gourry@gourry.net>]

On Thu, Jan 22, 2026 at 09:32:51AM +0900, Akinobu Mita wrote:
> Almost all of the execution time is consumed by folio_alloc_swap(),
> and analysis using Flame Graph reveals that spinlock contention is
> occurring in the call path __mem_cgroup_try_charge_swap ->
> __memcg_memory_event -> cgroup_file_notify.
> 
> In this reproduction procedure, no swap is configured, and calls to
> folio_alloc_swap() always fail. To avoid spinlock contention, I tried
> modifying the source code to return -ENOMEM without calling
> folio_alloc_swap(), but this caused other lock contention
> (lruvec->lru_lock in evict_folios()) in several other places, so it
> did not work around the problem.

Doesn't this suggest what I mentioned earlier?  If you don't demote when
the target node is full, then you're removing a memory pressure signal
from the lower node and reclaim won't ever clean up the lower node to
make room for future demotions.

I might be missing something here, though, is your system completely out
of memory at this point?

Presumably you're hitting direct reclaim and not just waking up kswapd
because things are locking up.

If there's no swap and no where to demote, then this all sounds like
normal OOM behavior.

Does this whole thing go away if you configure some swap space?

> 
> When demotion_enabled is true, if there is no free memory on the target
> node during memory allocation, even if there is no swap device, demotion
> may be able to move anonymous pages to a lower node and free up memory,
> so more anonymous pages become candidates for eviction.
> However, if free memory on the target node for demotion runs out,
> various processes will perform similar operations in search of free
> memory, wasting time on lock contention.
> 
> Reducing lock contention or changing the eviction process is also an
> interesting solution, but at present I have not come up with any workaround
> other than disabling demotion when free memory on lower-level nodes is
> exhausted.

The lock contention seems like a symptom, not the cause.  The cause
appears to be that you're out of memory with no swap configured.

~Gregory