RE: [PATCH 0/3] mm/lru_gen: move lru_gen control interface from debugfs to procfs

[wangzicheng]

Hi Barry,

> Then please check with Suren whether it is possible to backport this to
> the Android common kernel.
> My understanding is that this should already be present in the Android 6.12
> kernel.
> 
Thanks for the reminding.

> >
> > Since the max swappiness is 200, there are quite scenarios that file
> > pages are the only option.
> >
> > Quote from kairui's reply:
> > > Right, we are seeing similar problems on our server too. To workaround
> > > it we force an age iteration before reclaiming when it happens, which
> > > isn't the best choice. When the LRU is long and the opposite type of
> > > the folios we want to reclaim is piling up in the oldest gen, a forced
> > > age will have to move all these folios, which leads to long tailing
> > > issues. Let's work on a reasonable solution for that.
> >
> 
> We all agree that MGLRU has this generation issue. You mentioned it, I
> agreed
> and noted that both Kairui and I had observed it. Then Kairui replied that he
> had indeed seen it as well. Now you are using Kairui’s reply to argue against
> me, and I honestly don’t understand the logic behind your responses.
> 

My apologize if my previous wording caused any confusion.

The only thing the patchset (want to) do is forcing 2/3 gens aging right before proactive
reclaim, and it helps reclaim more anon pages and preserve more file pages under
certain workload. (400~800MB MemAvailable improvement).

The reason for quoting Kairui's reply:
`force aging 2/3 gens before reclaim` would be roughly similar in spirit to what Kairui
referred to ` force an age iteration before reclaiming`, from my understanding.

If my understanding is inaccurate, please feel free to correct me.

> > Again, thank you for your guidance. We will carefully evaluate the
> > Patchset[1] you recommended.
> >
> > > Hi Zicheng,
> > >
> > > On Mon, Dec 1, 2025 at 5:55 PM wangzicheng <wangzicheng@xxxxxxxxx>
> > > wrote:
> > > >
> > > > Hi Barry,
> > > >
> > > > Thank you for the comment, actually we do know the cgroup file.
> > > >
> > > > What we really need is to *proactive aging 2~3 gens* before proactive
> > > reclaim.
> > > > (especially after cold launches when no anon pages in the oldest gens)
> > > >
> > > > The proactive aging also helps distribute the anon and file pages evenly
> in
> > > > MGLRU gens. And reclaiming won't fall into file caches.
> > >
> > > I’m not quite sure what you mean by “reclaiming won’t fall into file
> caches.”
> > >
> > > I assume you mean you configured a high swappiness for MGLRU
> proactive
> > > reclamation, so when both anon and file have four generations,
> > > `get_type_to_scan()` effectively always returns anon?
> > >
> > > >
> > > > > Also note that memcg already has an interface for proactive
> reclamation,
> > > > > so I’m not certain whether your patchset can coexist with it or extend
> > > > > it to meet your requirements—which seems quite impossible to me
> > > > >
> > > > > memory.reclaim
> > > > >         A write-only nested-keyed file which exists for all cgroups.
> > > > >
> > > > >         This is a simple interface to trigger memory reclaim in the
> > > > >         target cgroup.
> > > > >
> > > > >         Example::
> > > > >
> > > > >           echo "1G" > memory.reclaim
> > > > >
> > > > >         Please note that the kernel can over or under reclaim from
> > > > >         the target cgroup. If less bytes are reclaimed than the
> > > > >         specified amount, -EAGAIN is returned.
> > > > >
> > > > This remind me that adding a `memor.aging` under memcg directories
> > > > rather than adding new procfs files is also a great option.
> > >
> > > I still don’t understand why. Aging is something MGLRU itself should
> > > handle; components outside MGLRU, such as cgroup v2, do not need to
> be
> > > aware of this concept at all. Exposing it will likely lead to another
> > > immediate NAK.
> > >
> > > In short, aging should remain within MGLRU’s internal scope.
> >
> > I would like to express a different point of view. We are working on
> something
> > Interesting on it, will be shared once ready.
> 
> You are always welcome to share, but please understand that memory.aging
> is
> not of interest to any module outside the scope of MGLRU itself. An
> interface
> is an interface, and internal implementation should remain internal. In other
> words, there is no reason for cgroupv2 to be aware of what “aging” is.
> 
> You may submit your new code as a "fix" for the generation issue without
> introducing a new interface. That would be a good starting point for
> discussing how to resolve the problem.
> 

Completely agree with your guidance.
We will revisit the design and think about the next version, and try to keep the
mechanism internally.

> >
> > >
> > > But it seems you do want some policy control for your proactive
> > > reclamation, such as always reclaiming anon pages or reclaiming them
> > > more aggressively than file pages. I assume Zhongkun’s patch [1] we
> > > mentioned earlier should provide support for that, correct?
> > >
> > > As a workaround, you can set `swappiness=max` for `memory.reclaim`
> > > before
> > > we internally improve the handling of the aging issue. In short,
> > > “proactive aging” and similar mechanisms should be handled
> automatically
> > > and internally within the scope of the MGLRU code.
> >
> > Sure, we will make a careful evaluation.
> 
> Thanks
> Barry

Best,
Zicheng