Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

[Michal Hocko]

On Mon 05-12-22 18:34:05, Mina Almasry wrote:
> commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> reclaim"") enabled demotion in memcg reclaim, which is the right thing
> to do, however, it introduced a regression in the behavior of
> try_to_free_mem_cgroup_pages().
> 
> The callers of try_to_free_mem_cgroup_pages() expect it to attempt to
> reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage
> of the cgroup should reduce by nr_pages. The callers expect
> try_to_free_mem_cgroup_pages() to also return the number of pages
> reclaimed, not demoted.
> 
> However, what try_to_free_mem_cgroup_pages() actually does is it
> unconditionally counts demoted pages as reclaimed pages. So in practice
> when it is called it will often demote nr_pages and return the number of
> demoted pages to the caller. Demoted pages don't lower the memcg usage,
> and so try_to_free_mem_cgroup_pages() is not actually doing what the
> callers want it to do.
> 
> Various things work suboptimally on memory tiered systems or don't work
> at all due to this:
> 
> - memory.high enforcement likely doesn't work (it just demotes nr_pages
>   instead of lowering the memcg usage by nr_pages).
> - try_charge_memcg() will keep retrying the charge while
>   try_to_free_mem_cgroup_pages() is just demoting pages and not actually
>   making any room for the charge.

This has been brought up during the review https://lore.kernel.org/all/YoYTEDD+c4GT0xYY@xxxxxxxxxxxxxx/

> - memory.reclaim has a wonky interface. It advertises to the user it
>   reclaims the provided amount but it will actually often demote that
>   amount.
> 
> There may be more effects to this issue.
> 
> To fix these issues I propose shrink_folio_list() to only count pages
> demoted from inside of sc->nodemask to outside of sc->nodemask as
> 'reclaimed'.

Could you expand on why the node mask matters? From the charge point of
view it should be completely uninteresting as the charge remains.

I suspect we really need to change to reclaim metrics for memcg reclaim.
In the memory balancing reclaim we can indeed consider demotions as a
reclaim because the memory is freed in the end but for the memcg reclaim
we really should be counting discharges instead. No demotion/migration will
free up charges.
-- 
Michal Hocko
SUSE Labs