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 29, 2026 at 09:51:44AM +0900, Akinobu Mita wrote:
> > I'm not sure there's a best-option here, we may need additional input to
> > determine what the least-worst option is.  Causing LRU inversion when
> > all the nodes are pressured but swap is available is not preferable.
> 
> Would it be better if can_demote() returned false after checking that
> there is no free swap space at all and that there is not enough free space
> on the demote target node or its lower nodes?
>

I need some time to think on this.

If we take your patch, I think we essentially default to the same
behavior as-if demotion was wholesale disabled in the first place -
toptier nodes would reclaim space directly into swap.  zswap would
probably get skipped if we're already in direct reclaim (if we can't
allocate a page, neither can zswap).

The alternative is reclaim makes absolutely no progress, even if there
is (z)swap space available - which is essentially what you're
experiencing.

Maybe there is an argument for simply falling back to swap if there's no
room on any node further away.

Will ponder this for a bit and get back to you.

~Gregory