Message-ID: <Z9Rs8ZtgkupXpFYn@google.com>
Date: Fri, 14 Mar 2025 17:52:49 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.com>,
	Zhongkun He <hezhongkun.hzk@...edance.com>,
	akpm@...ux-foundation.org, muchun.song@...ux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm: vmscan: skip the file folios in proactive reclaim
 if swappiness is MAX

On Fri, Mar 14, 2025 at 12:57:39PM -0400, Johannes Weiner wrote:
> On Fri, Mar 14, 2025 at 03:49:30PM +0100, Michal Hocko wrote:
> > On Fri 14-03-25 10:18:33, Johannes Weiner wrote:
> > > On Fri, Mar 14, 2025 at 10:27:57AM +0100, Michal Hocko wrote:
> > [...]
> > > > I have just noticed that you have followed up [1] with a concern that
> > > > using swappiness in the whole min-max range without any heuristics turns
> > > > out to be harder than just relying on the min and max as extremes.
> > > > What seems to be still missing (or maybe it is just me not seeing that)
> > > > is why should we only enforce those extreme ends of the range and still
> > > > preserve under-defined semantic for all other swappiness values in the
> > > > pro-active reclaim.
> > > 
> > > I'm guess I'm not seeing the "under-defined" part.
> > 
> > What I meant here is that any other value than both ends of swappiness
> > doesn't have generally predictable behavior unless you know specific
> > details of the current memory reclaim heuristics in get_scan_count.
> > 
> > > cache_trim_mode is
> > > there to make sure a streaming file access pattern doesn't cause
> > > swapping.
> > 
> > Yes, I am aware of the purpose.
> > 
> > > He has a special usecase to override cache_trim_mode when he
> > > knows a large amount of anon is going cold. There is no way we can
> > > generally remove it from proactive reclaim.
> > 
> > I believe I do understand the requirement here. The patch offers
> > counterpart to noswap pro-active reclaim and I do not have objections to
> > that.
> > 
> > The reason I brought this up is that everything in between 0..200 is
> > kinda gray area. We've had several queries why swappiness=N doesn't work
> > as expected and the usual answer was because of heuristics. Most people
> > just learned to live with that and stopped fine tuning vm_swappiness.
> > Which is good I guess.
> 
> You're still oversimplifying and then dismissing. The heuristics don't
> make swappiness meaningless, they make it useful in the first place.
> 
>   This control is used to define the rough relative IO cost of swapping
>   and filesystem paging, as a value between 0 and 200.
> 
> This is clearly defined, and implemented as such. cache_trim_mode is
> predicated on the *absence* of paging and caching benefits: A linear,
> use-once file access pattern that *does not* benefit from additional
> cache space. Kicking out anon for that purpose would be wrong under
> pretty much any circumstance. That's why it "overrides" swappiness:
> swappiness cannot apply when swapping at all would be nonsense.
> 
> Proactive reclaimers like ours rely on this. We use swappiness to
> express exactly what it says on the tin: the relative cost between
> thrashing file vs anon. We use it quite effectively to manage anon
> write rates for flash wear management e.g. Obviously that doesn't mean
> we want to swap when somebody streams through a large file set.
> 
> Zhongkun's case is a significant exception. He just wants to get rid
> of known-cold anon set. This level of insight into userspace access
> patterns is rare in practice. You could argue that MADV_PAGEOUT might
> be more suitable for that.

We have a similar use case at Google where we have a known-cold anon set
and we proactively reclaim it. This is why we previously proposed
type=anon/file/.., but swappiness is more flexible for use cases like
the one Johannes describes above.

> But I also don't necessarily see a problem
> with making swappiness=200 do it; although we might have to teach our
> proactive reclaimer to auto-tune between 1 and 199 then.

Would it be better if we don't use the existing swappiness=200 for this?

We can support something like memory.reclaim X swappiness=max instead to
achieve the "anon only" mode without affecting the existing semantics of
swappiness at all. I have a feeling I may have already proposed that at
some point.

In the kernel, we can define a new value (say 201 or 1000) that means
anon only and set it in memory_reclaim() when "max" is specified. We can
then explicitly check for this value in get_scan_count() (we probably
also need to handle MGLRU?).

>From a user perspective the swappiness semantics remain unchanged, and
you do not need to teach your proactive reclaim to auto tune up to 199
of 200. We just support a new swappiness mode specific to proactive
reclaim.

WDYT?

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