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

[Johannes Weiner]

Hello Ying,

On Thu, Nov 24, 2022 at 01:51:20PM +0800, Huang, Ying wrote:
> 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
> 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.

If they're hotter than reclaim candidates on the toptier, shouldn't
they get promoted instead and make room that way? We may have to tweak
the watermark logic a bit to facilitate that (allow promotions where
regular allocations already fail?). But this sort of resorting would
be preferable to age inversions.

The mlock scenario sounds possible. In that case, it wouldn't be an
aging inversion, since there is nothing colder on the CXL node.

Maybe a bypass check should explicitly consult the demotion target
watermarks against its evictable pages (similar to the file_is_tiny
check in prepare_scan_count)?

Because in any other scenario, if there is a bug in the promo/demo
coordination, I think we'd rather have the OOM than deal with age
inversions causing intermittent performance issues that are incredibly
hard to track down.