Re: [RFC PATCH V1] mm: Disable demotion from proactive reclaim

[Mina Almasry]

On Wed, Nov 23, 2022 at 9:52 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Hi, Johannes,
>
> Johannes Weiner <hannes@xxxxxxxxxxx> writes:
> [...]
> >
> > The fallback to reclaim actually strikes me as wrong.
> >
> > Think of reclaim as 'demoting' the pages to the storage tier. If we
> > have a RAM -> CXL -> storage hierarchy, we should demote from RAM to
> > CXL and from CXL to storage. If we reclaim a page from RAM, it means
> > we 'demote' it directly from RAM to storage, bypassing potentially a
> > huge amount of pages colder than it in CXL. That doesn't seem right.
> >
> > If demotion fails, IMO it shouldn't satisfy the reclaim request by
> > breaking the layering. Rather it should deflect that pressure to the
> > lower layers to make room. This makes sure we maintain an aging
> > pipeline that honors the memory tier hierarchy.
>
> Yes.  I think that we should avoid to fall back to reclaim as much as
> possible too.  Now, when we allocate memory for demotion
> (alloc_demote_page()), __GFP_KSWAPD_RECLAIM is used.  So, we will trigger

I may be missing something but as far I can tell reclaim is disabled
for allocations from lower tier memory:
https://elixir.bootlin.com/linux/v6.1-rc7/source/mm/vmscan.c#L1583

I think this is maybe a good thing when doing proactive demotion. In
this case we probably don't want to try to reclaim from lower tier
nodes and instead fail the proactive demotion. However I can see this
being desirable when the top tier nodes are under real memory pressure
to deflect that pressure to the lower tier nodes.

> kswapd reclaim on lower tier node to free some memory to avoid fall back
> to reclaim on current (higher tier) node.  This may be not good enough,
> for example, the following patch from Hasan may help via waking up
> kswapd earlier.
>
> https://lore.kernel.org/linux-mm/b45b9bf7cd3e21bca61d82dcd1eb692cd32c122c.1637778851.git.hasanalmaruf@xxxxxx/
>
> Do you know what is the next step plan for this patch?
>
> Should we do even more?
>
> From another point of view, I still think that we can use falling back
> to reclaim as the last resort to avoid OOM in some special situations,
> for example, most pages in the lowest tier node are mlock() or too hot
> to be reclaimed.
>
> > So I'm hesitant to design cgroup controls around the current behavior.

I sent RFC v2 patch:
https://lore.kernel.org/linux-mm/20221130020328.1009347-1-almasrymina@xxxxxxxxxx/T/#u

Please take a look when convenient. Thanks!

> >
>
> Best Regards,
> Huang, Ying
>