Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap

From: Chris Li <chrisl@kernel.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>,
	lsf-pc@lists.linux-foundation.org,
	Johannes Weiner <hannes@cmpxchg.org>,
	Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>,
	Shakeel Butt <shakeelb@google.com>,
	David Rientjes <rientjes@google.com>,
	Hugh Dickins <hughd@google.com>,
	Seth Jennings <sjenning@redhat.com>,
	Dan Streetman <ddstreet@ieee.org>,
	Vitaly Wool <vitaly.wool@konsulko.com>,
	Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>,
	Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>,
	Michal Hocko <mhocko@suse.com>, Wei Xu <weixugc@google.com>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Tue, 28 Mar 2023 14:32:06 -0700	[thread overview]
Message-ID: <ZCNc1nTOArp6OQ3G@google.com> (raw)
In-Reply-To: <CAJD7tkaGDBxO2q3X5oQj+C11z3mHdFK_Dw0anFPWjWPp99ra7g@mail.gmail.com>

On Tue, Mar 28, 2023 at 02:01:09PM -0700, Yosry Ahmed wrote:
> On Tue, Mar 28, 2023 at 1:50 PM Chris Li <chrisl@kernel.org> wrote:
> >
> > On Tue, Mar 28, 2023 at 12:59:31AM -0700, Yosry Ahmed wrote:
> > > > > I don't have a problem with this approach, it is not really clean as
> > > > > we still treat zswap as a swapfile and have to deal with a lot of
> > > > > unnecessary code like swap slots handling and whatnot.
> > > >
> > > > These are existing code?
> >
> > Yes. The ghost swap file are existing code used in Google for many years.
> >
> > > I was referring to the fact that today with zswap being tied to
> > > swapfiles we do some necessary work such as searching for swap slots
> > > during swapout. The initial swap_desc approach aimed to avoid that.
> > > With this minimal ghost swapfile approach we retain this unfavorable
> > > behavior.
> >
> > Can you explain how you can avoid the free swap entry search
> > in the swap descriptor world?
> 
> For zswap, in the swap descriptor world, you just need to allocate a
> struct zswap_entry and have the swap descriptor point to it. No need
> for swap slot management since we are not tied to a swapfile and pages
> in zswap do not have a specific position.

Your swap descriptor will be using one swp_entry_t, which get from the PTE 
to lookup, right? That is the swap entry I am talking about. You just
substitute zswap swap entry with the swap descriptor swap entry.
You still need to allocate from the free swap entry space at least once.

Chris