linux-kernel - Re: [External] Re: [RFC PATCH 0/2] Add disable_unmap

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Message-ID: <ZtCNA3iKd0_mH8Bf@tiehlicka>
Date: Thu, 29 Aug 2024 17:00:19 +0200
From: Michal Hocko <mhocko@...e.com>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev, muchun.song@...ux.dev,
	lizefan.x@...edance.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [External] Re: [RFC PATCH 0/2] Add disable_unmap_file arg to
 memory.reclaim

On Thu 29-08-24 22:30:09, Zhongkun He wrote:
> On Thu, Aug 29, 2024 at 9:36 PM Michal Hocko <mhocko@...e.com> wrote:
[...]
> > Seeing this my main question is whether we should focus on swappiness
> > behavior more than adding a very strange and very targetted reclaim
> > mode. After all we have a mapped memory and executables protection in
> > place. So in the end this is more about balance between anon vs. file
> > LRUs.
> >
> 
> I  have a question about the swappiness, if set the swappiness=0, we can only
> reclaim the file pages. but we do not have an option to disable the reclaim from
> file pages because there are faster storages for the swap without IO, like zram
> and zswap.  I wonder if we can give it a try in this direction.

I do not think we should give any guarantee that 200 will only reclaim
anon pages. But having that heavily anon oriented makes sense and I
thought this was an existing semantic.

[...]
> > > The delay of the task becomes more serious because reading data will
> > > be slower.  Hot pages will thrash repeatedly between the memory and
> > > the disk.
> >
> > Doesn't refault stats and IO cost aspect of the reclaim when balancing
> > LRUs dealing with this situation already? Why it doesn't work in your
> > case? Have you tried to investigate that?
> 
> OK, I'll try to reproduce the problem again. but IIUC, we could not reclaim
> pages from one side. Please see this 'commit d483a5dd009  ("mm:
> vmscan: limit the range of LRU type balancing")'  [1]
> 
> Unless this condition is met:
> sc->file_is_tiny =
>             file + free <= total_high_wmark &&
>             !(sc->may_deactivate & DEACTIVATE_ANON) &&
>             anon >> sc->priority;

There have been some changes in this area where swappiness was treated
differently so it would make sense to investigate with the current mm
tree.

> [1]: https://lore.kernel.org/all/20200520232525.798933-15-hannes@cmpxchg.org/T/#u
> 
> > --
> > Michal Hocko
> > SUSE Labs

-- 
Michal Hocko
SUSE Labs