linux-kernel - RE: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap

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Message-ID: <SJ0PR11MB5678773A3A8AD10EE584725EC96A2@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Thu, 26 Sep 2024 19:40:37 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.com>, Johannes Weiner <hannes@...xchg.org>
CC: "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 6/8] mm: zswap: Support mTHP swapout in zswap_store().

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@...gle.com>
> Sent: Thursday, September 26, 2024 11:46 AM
> To: Johannes Weiner <hannes@...xchg.org>
> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@...el.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 6/8] mm: zswap: Support mTHP swapout in
> zswap_store().
> 
> On Thu, Sep 26, 2024 at 11:43 AM Johannes Weiner <hannes@...xchg.org>
> wrote:
> >
> > On Thu, Sep 26, 2024 at 05:29:30PM +0000, Sridhar, Kanchana P wrote:
> > > > > 3) Keep the approach in v7 where obj_cgroup_get/put is localized to
> > > > >     zswap_store_page for both success and error conditions, and any
> > > > unwinding
> > > > >     state in zswap_store will take care of dropping references obtained
> from
> > > > >     prior successful writes (from this or prior invocations of
> zswap_store).
> > > >
> > > > I am also fine with doing that and doing the reference batching as a
> follow up.
> > >
> > > I think so too! We could try and improve upon (3) with reference batching
> > > in a follow-up patch.
> >
> > Yeah, I agree. The percpu-refcounts are not that expensive, we should
> > be able to live with per-page ops for now.
> >
> > One thing you *can* do from the start is tryget a pool reference in
> > zswap_store(), to prevent the pools untimely demise while you work on
> > it, and then in zswap_store_page() you can do gets instead of trygets.
> >
> > You'd have to rename zswap_pool_get() to zswap_pool_tryget() (which is
> > probably for the best) and implement the trivial new zswap_pool_get().
> 
> Yeah I was actually planning to send a follow-up patch to do exactly
> that until we figure out proper patching for the refcounts. Even
> better if Kanchana incorporates it in the next version :)

Sure, Yosry, I will incorporate it in the next version!

Thanks again,
Kanchana