linux-kernel - Re: [PATCH v9 6/7] mm: zswap: Support large folios in zswap

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Message-ID: <CAJD7tkZk=9Su3vtdFcghD0R+GqSchn5b1L91SgTvxcX79rmdOA@mail.gmail.com>
Date: Tue, 1 Oct 2024 10:01:26 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"hannes@...xchg.org" <hannes@...xchg.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>, 
	"willy@...radead.org" <willy@...radead.org>, "Zou, Nanhai" <nanhai.zou@...el.com>, 
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v9 6/7] mm: zswap: Support large folios in zswap_store().

On Tue, Oct 1, 2024 at 9:58 AM Sridhar, Kanchana P
<kanchana.p.sridhar@...el.com> wrote:
>
> > -----Original Message-----
> > From: Yosry Ahmed <yosryahmed@...gle.com>
> > Sent: Monday, September 30, 2024 11:00 PM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> > Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> > hannes@...xchg.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; willy@...radead.org; Zou, Nanhai <nanhai.zou@...el.com>;
> > Feghali, Wajdi K <wajdi.k.feghali@...el.com>; Gopal, Vinodh
> > <vinodh.gopal@...el.com>
> > Subject: Re: [PATCH v9 6/7] mm: zswap: Support large folios in zswap_store().
> >
> > [..]
> > > > >  store_failed:
> > > > >         zpool_free(entry->pool->zpool, entry->handle);
> > > > > -put_pool:
> > > > > -       zswap_pool_put(entry->pool);
> > > > > -freepage:
> > > > > +put_pool_objcg:
> > > > > +       zswap_pool_put(pool);
> > > > > +       obj_cgroup_put(objcg);
> > > >
> > > > I think if we reorder the function we can drop these calls, make the
> > > > comments positioned a bit better, and centralize the entry
> > > > initializations. I am also not a fan of passing a semi-initialized
> > > > entry to zswap_compress() to get the pool pointer.
> > > >
> > > > Does the following diff improve things or did I miss something?
> > >
> > > We shouldn’t be adding the entry to the xarray before initializing its pool
> > > and objcg, right? Please let me know if I am misunderstanding what you're
> > > proposing in the diff.
> >
> > It should be safe. We already initialize entry->lru after we insert
> > the entry in the tree. See the comment above the call to
> > zswap_lru_add(). Basically we are protected against concurrent
> > stores/loads through the folio lock, and are protected against
> > writeback because the entry is not on the LRU yet.
>
> Thanks for the clarification, Yosry. Since this is a change in the entry
> initialization wrt the mainline, is it Ok if this is done in a follow-up patch?

Sure. We can discuss it separately. Do you want me to send a patch or
do you intend to?