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: <CAJD7tkbRewK-L07GPzX6YtBnw7zpA1NboBCEyszoT4=-PvpdYw@mail.gmail.com>
Date: Tue, 24 Sep 2024 17:43:22 -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>, 
	"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.