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

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Message-ID: <SJ0PR11MB5678300E878F43C5BB8C485DC96A2@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Thu, 26 Sep 2024 19:39:39 +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 6/8] mm: zswap: Support mTHP swapout in zswap_store().

> -----Original Message-----
> From: Johannes Weiner <hannes@...xchg.org>
> Sent: Thursday, September 26, 2024 11:43 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 6/8] mm: zswap: Support mTHP swapout in
> zswap_store().
> 
> 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.

Sure, this sounds good Johannes, thanks for the suggestion! I already
do a zswap_pool_current_get() at the beginning of zswap_store in the
v7 code, for this purpose.

> 
> 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().

Ok, will do so.

Thanks,
Kanchana