linux-kernel - Re: [PATCH v7 5/8] mm: zswap: Compress and store a specific page in a folio.

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Message-ID: <20240925135348.GB875661@cmpxchg.org>
Date: Wed, 25 Sep 2024 09:53:48 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nphamcs@...il.com" <nphamcs@...il.com>,
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>,
	"shakeel.butt@...ux.dev" <shakeel.butt@...ux.dev>,
	"ryan.roberts@....com" <ryan.roberts@....com>,
	"Huang, Ying" <ying.huang@...el.com>,
	"21cnbao@...il.com" <21cnbao@...il.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Zou, Nanhai" <nanhai.zou@...el.com>,
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>,
	"Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v7 5/8] mm: zswap: Compress and store a specific page in
 a folio.

On Wed, Sep 25, 2024 at 01:49:03AM +0000, Sridhar, Kanchana P wrote:
> > From: Yosry Ahmed <yosryahmed@...gle.com>
> > I think it's more correct and efficient to update the atomic once
> > after all the pages are successfully compressed and stored.
> 
> Actually this would need to co-relate with the limits checking strategy,
> because the atomic is used there and needs to be as accurate as possible.

For the limit checks, we use the zpool counters, not zswap_stored_pages.

zswap_stored_pages is used in the zswap shrinker to guesstimate
pressure, so it's likely a good thing to only count entries that are
expected to stay, and not account the ones that might fail just yet.

> As far as the vmstat 'zswpout', the reason I left it as-is in my patchset
> was to be more indicative of the actual zswpout compute events that
> occurred (for things like getting the compressions count), regardless
> of whether or not the overall mTHP store was successful. If this vmstat
> needs to reflect only successful zswpout events (i.e., represent the zswap
> usage), I can fix it by updating it once only if the mTHP is stored successfully.

Yeah, that's fine as well.

I would suggest batching them both at the end of zswap_store().