Re: [PATCH 0/3] mm,TPP: Enable promotion of unmapped pagecache

[Gregory Price]

On Tue, Nov 05, 2024 at 10:00:59AM +0800, Huang, Ying wrote:
> Hi, Gregory,
> >> 
> >> Several years ago, we have tried to use the access time tracking
> >> mechanism of NUMA balancing to track the access time latency of unmapped
> >> file cache folios.  The original implementation is as follows,
> >> 
> >> https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/commit/?h=tiering-0.8&id=5f2e64ce75c0322602c2ec8c70b64bb69b1f1329
> >> 
> >> What do you think about this?
> >> 
> >
> > Coming back around to explore this topic a bit more, dug into this old
> > patch and the LRU patch by Keith - I'm struggling find a good option
> > that doesn't over-complicate or propose something contentious.
> >
> >
> > I did a browse through lore and did not see any discussion on this patch
> > or on Keith's LRU patch, so i presume discussion on this happened largely
> > off-list.  So if you have any context as to why this wasn't RFC'd officially
> > I would like more information.
> 
> Thanks for doing this.  There's no much discussion offline.  We just
> don't have enough time to work on the solution.
> 

Exploring and testing this a little further, I brought this up to current
folio work in 6.9 and found this solution to be unstable as-is.

After some work to fix lock/reference issues, Johannes pointed out that
__filemap_get_folio can be called from an atomic context - which means it
may not be safe to do migrations in this context.

We're back to looking at something like an LRU-esque system, but now we're
thinking about isolating the folios in folio_mark_accessed into a task-local
list, and then process the list on resume.

Basically we're thinking

1) hook folio_mark_accessed and use PG_ACTIVE/PG_ACCESSED to determine whether
   the page is a promotion candidate.
2) if it is, isolate it from the LRU - which is safe because folio_mark_accessed
   already does this elsewhere, and place it onto current->promo_queue
3) set_notify_resume
4) add logic to resume_user_mode_work() to run through current->promo_queue and
   either promote the pages accordingly, or do folio_putback_lru on failure.

Going to RFC this up

~Gregory