linux-kernel - RE: [PATCH v7 4/8] mm: zswap: Refactor code to delete stored offsets in case of errors.

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Message-ID: <SJ0PR11MB56785A70F28FB3A7B6A644EFC9692@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Wed, 25 Sep 2024 01:18:15 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosryahmed@...gle.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>,
	"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 4/8] mm: zswap: Refactor code to delete stored offsets
 in case of errors.

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@...gle.com>
> Sent: Tuesday, September 24, 2024 5:43 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; Zou, Nanhai <nanhai.zou@...el.com>; Feghali, Wajdi K
> <wajdi.k.feghali@...el.com>; Gopal, Vinodh <vinodh.gopal@...el.com>
> Subject: Re: [PATCH v7 4/8] mm: zswap: Refactor code to delete stored
> offsets in case of errors.
> 
> On Tue, Sep 24, 2024 at 3:33 PM Sridhar, Kanchana P
> <kanchana.p.sridhar@...el.com> wrote:
> >
> > > -----Original Message-----
> > > From: Yosry Ahmed <yosryahmed@...gle.com>
> > > Sent: Tuesday, September 24, 2024 12:20 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; Zou, Nanhai <nanhai.zou@...el.com>; Feghali, Wajdi K
> > > <wajdi.k.feghali@...el.com>; Gopal, Vinodh <vinodh.gopal@...el.com>
> > > Subject: Re: [PATCH v7 4/8] mm: zswap: Refactor code to delete stored
> > > offsets in case of errors.
> > >
> > > On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
> > > <kanchana.p.sridhar@...el.com> wrote:
> > > >
> > > > Added a new procedure zswap_delete_stored_offsets() that can be
> > > > called to delete stored offsets in a folio in case zswap_store()
> > > > fails or zswap is disabled.
> > >
> > > I don't see the value in this helper. It will get called in one place
> > > AFAICT, and it is a bit inconsistent that we have to explicitly loop
> > > in zswap_store() to store pages, but the loop to delete pages upon
> > > failure is hidden in the helper.
> > >
> > > I am not against adding a trivial zswap_tree_delete() helper (or
> > > similar) that calls xa_erase() and  zswap_entry_free() to match
> > > zswap_tree_store() if you prefer that.
> >
> > This is a good point. I had refactored this routine in the context
> > of my code that does batching and the same loop over the mTHP's
> > subpages would get called in multiple error condition cases.
> >
> > I am thinking it might probably make sense for say zswap_tree_delete()
> > to take a "folio" and "tree" and encapsulate deleting all stored offsets
> > for that folio. Since we have already done the computes for finding the
> > "tree", having that as an input parameter is mainly for latency, but if
> > it is cleaner to have "zswap_tree_delete(struct folio *folio)", that should
> > be Ok too. Please let me know your suggestion on this.
> >
> 
> What I meant is "zswap_tree_delete(struct xarray *tree, pgoff_t
> offset)", and loop and call this  in zswap_store(). This would be
> consistent on looping and calling zswap_store_page().
> 
> But we can keep the helper as-is actually and just rename it to
> zswap_tree_delete() and move the loop inside. No strong preference.

Ok, sounds good.

Thanks,
Kanchana