linux-kernel - Re: [PATCH v2 0/2] mm: swap: mTHP swap allocator base on swap cluster order

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Date: Fri, 14 Jun 2024 19:59:21 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Chris Li <chrisl@...nel.org>
Cc: Kairui Song <kasong@...cent.com>, Ryan Roberts <ryan.roberts@....com>,
 "Huang, Ying" <ying.huang@...el.com>, Kalesh Singh
 <kaleshsingh@...gle.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Barry Song <baohua@...nel.org>
Subject: Re: [PATCH v2 0/2] mm: swap: mTHP swap allocator base on swap
 cluster order

On Fri, 14 Jun 2024 19:51:11 -0700 Chris Li <chrisl@...nel.org> wrote:

> > I'm having trouble understanding the overall impact of this on users.
> > We fail the mTHP swap allocation and fall back, but things continue to
> > operate OK?
> 
> Continue to operate OK in the sense that the mTHP will have to split
> into 4K pages before the swap out, aka the fall back. The swap out and
> swap in can continue to work as 4K pages, not as the mTHP. Due to the
> fallback, the mTHP based zsmalloc compression with 64K buffer will not
> happen. That is the effect of the fallback. But mTHP swap out and swap
> in is relatively new, it is not really a regression.

Sure, but it's pretty bad to merge a new feature only to have it
ineffective after a few hours use.

> >
> > > There is some test number in the V1 thread of this series:
> > > https://lore.kernel.org/r/20240524-swap-allocator-v1-0-47861b423b26@kernel.org
> >
> > Well, please let's get the latest numbers into the latest patchset.
> > Along with a higher-level (and quantitative) description of the user impact.
> 
> I will need Barray's help to collect the number. I don't have the
> setup to reproduce his test result.
> Maybe a follow up commit message amendment for the test number when I get it?

Yep, I alter changelogs all the time.

> >
> > I'll add this into mm-unstable now for some exposure, but at this point
> > I'm not able to determine whether it should go in as a hotfix for
> > 6.10-rcX.
> 
> Maybe not need to be a hotfix. Not all Barry's mTHP swap out and swap
> in patch got merged yet.

OK, well please let's give appropriate consideration to what we should
add to 6.10-rcX in order to have this feature working well.