Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

Yosry Ahmed
2023-11-22T10:39:59Z

On Wed, Nov 22, 2023 at 2:09 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 22-11-23 09:52:42, Michal Hocko wrote:
> > On Tue 21-11-23 22:44:32, Yosry Ahmed wrote:
> > > On Tue, Nov 21, 2023 at 10:41 PM Liu Shixin <liushixin2@huawei.com> wrote:
> > > >
> > > >
> > > > On 2023/11/21 21:00, Michal Hocko wrote:
> > > > > On Tue 21-11-23 17:06:24, Liu Shixin wrote:
> > > > >
> > > > > However, in swapcache_only mode, the scan count still increased when scan
> > > > > non-swapcache pages because there are large number of non-swapcache pages
> > > > > and rare swapcache pages in swapcache_only mode, and if the non-swapcache
> > > > > is skipped and do not count, the scan of pages in isolate_lru_folios() can
> > > > > eventually lead to hung task, just as Sachin reported [2].
> > > > > I find this paragraph really confusing! I guess what you meant to say is
> > > > > that a real swapcache_only is problematic because it can end up not
> > > > > making any progress, correct?
> > > > This paragraph is going to explain why checking swapcache_only after scan += nr_pages;
> > > > >
> > > > > AFAIU you have addressed that problem by making swapcache_only anon LRU
> > > > > specific, right? That would be certainly more robust as you can still
> > > > > reclaim from file LRUs. I cannot say I like that because swapcache_only
> > > > > is a bit confusing and I do not think we want to grow more special
> > > > > purpose reclaim types. Would it be possible/reasonable to instead put
> > > > > swapcache pages on the file LRU instead?
> > > > It looks like a good idea, but I'm not sure if it's possible. I can try it, is there anything to
> > > > pay attention to?
> > >
> > > I think this might be more intrusive than we think. Every time a page
> > > is added to or removed from the swap cache, we will need to move it
> > > between LRUs. All pages on the anon LRU will need to go through the
> > > file LRU before being reclaimed. I think this might be too big of a
> > > change to achieve this patch's goal.
> >
> > TBH I am not really sure how complex that might turn out to be.
> > Swapcache tends to be full of subtle issues. So you might be right but
> > it would be better to know _why_ this is not possible before we end up
> > phising for couple of swapcache pages on potentially huge anon LRU to
> > isolate them. Think of TB sized machines in this context.
>
> Forgot to mention that it is not really far fetched from comparing this
> to MADV_FREE pages. Those are anonymous but we do not want to keep them
> on anon LRU because we want to age them indepdendent on the swap
> availability as they are just dropped during reclaim. Not too much
> different from swapcache pages. There are more constrains on those but
> fundamentally this is the same problem, no?

I agree it's not a first, but swap cache pages are more complicated
because they can go back and forth, unlike MADV_FREE pages which
usually go on a one way ticket AFAICT. Also pages going into the swap
cache can be much more common that MADV_FREE pages for a lot of
workloads. I am not sure how different reclaim heuristics will react
to such mobility between the LRUs, and the fact that all pages will
now only get evicted through the file LRU. The anon LRU will
essentially become an LRU that feeds the file LRU. Also, the more
pages we move between LRUs, the more ordering violations we introduce,
as we may put colder pages in front of hotter pages or vice versa.

All in all, I am not saying it's a bad idea or not possible, I am just
saying it's probably more complicated than MADV_FREE, and adding more
cases where pages move between LRUs could introduce problems (or make
existing problems more visible).

> --
> Michal Hocko
> SUSE Labs