linux-kernel - Re: [PATCH v4 0/4] mm: ZSWAP swap-out of mTHP folios

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Message-ID: <CAJD7tka1KcL7O+=YanStLz9VnsJtoKkB0d5SDyFUrezno0Z2Sg@mail.gmail.com>
Date: Fri, 23 Aug 2024 20:09:32 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, hannes@...xchg.org, ryan.roberts@....com, 
	ying.huang@...el.com, 21cnbao@...il.com, akpm@...ux-foundation.org, 
	nanhai.zou@...el.com, wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v4 0/4] mm: ZSWAP swap-out of mTHP folios

[..]
>
> I'm not trying to delay this patch - I fully believe in supporting
> zswap for larger pages (both mTHP and THP - whatever the memory
> reclaim subsystem throws at us).
>
> But we need to get to the bottom of this :) These are very suspicious
> and concerning data. If this is something urgent, I can live with a
> gate to enable/disable this, but I'd much prefer we understand what's
> going on here.

Agreed. I don't think merging this support is urgent, so I think we
should better understand what is happening here. If there is a problem
with how we charge compressed memory today (temporary double charges),
we need to sort this out before the the mTHP support, as it will only
make things worse.

I have to admit I didn't take a deep look at the discussion and data,
so there may be other problems that I didn't notice. It seems to me
like Kanchana is doing more debugging to understand what is happening,
so that's great!

As for the patches, we should sort out the impact on a higher level
before discussing implementation details. From a quick look though it
seems like the first patch can be dropped after Usama's patches that
remove the same-filled handling from zswap land, and the last two
patches can be squashed.