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

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Message-ID: <20240926184301.GA883850@cmpxchg.org>
Date: Thu, 26 Sep 2024 14:43:01 -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 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.

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().