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

[Huang, Ying]

Gregory Price <gourry@xxxxxxxxxx> writes:

> 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.

Sorry, I don't understand this, the above patch changes
filemap_get_pages() and grab_cache_page_write_begin() instead of
__filemap_get_folio().

> 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.

If necessary, we can use a similar method for above solution too.  And
we can filter accessed once folios with folio_mark_accessed() firstly.
That is, only promote a page if,

- record the folio access time in folio_mark_accessed() only
- when the folio are accessed again, and "access_time - record_time <
  threshold", promote the folio.

> 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.

Use a task_work?

> Going to RFC this up

--
Best Regards,
Huang, Ying