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

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Message-ID: <SJ0PR11MB5678DEAE7922ADAB8876F386C9692@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Wed, 25 Sep 2024 18:45:20 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Johannes Weiner <hannes@...xchg.org>
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>, "Sridhar, Kanchana P"
	<kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v7 5/8] mm: zswap: Compress and store a specific page in a
 folio.

> -----Original Message-----
> From: Johannes Weiner <hannes@...xchg.org>
> Sent: Wednesday, September 25, 2024 6:54 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: Yosry Ahmed <yosryahmed@...gle.com>; linux-kernel@...r.kernel.org;
> linux-mm@...ck.org; nphamcs@...il.com; chengming.zhou@...ux.dev;
> usamaarif642@...il.com; shakeel.butt@...ux.dev; ryan.roberts@....com;
> Huang, Ying <ying.huang@...el.com>; 21cnbao@...il.com; 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.

Thanks Johannes for your insights and comments. Yes, you are absolutely
right. My apologies.

> 
> 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.

Sure, makes sense.

> 
> > 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().

Ok, will do so in v8.

Thanks,
Kanchana