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: Fri, 24 Mar 2023 10:28:46 -0700	[thread overview]
Message-ID: <ZB3dzqasXxl9pk96@google.com> (raw)
In-Reply-To: <CAJD7tka=D=GbNasHG3noQNa0fwmbQsbELLBZoHZcXDQ2wLHYiA@mail.gmail.com>

On Thu, Mar 23, 2023 at 12:54:42PM -0700, Yosry Ahmed wrote:
> >
> > At the time of swap out page into A, we will not know if it will
> > move to B in a later time. I guess the swap ID xarray look up always
> > needs to be there?
> 
> If the indirection is enabled, yes.
> 
> >
> > > Moving a page from a swap backend A to another swap backend B should
> > > not be a problem in terms of the swap cache, as we will add it to the
> > > swap cache of B, modify the swap ID mapping to point to B, then remove
> > > it from the swap cache of A.
> >
> > That means when B swap in a page, it will always look up the swap ID
> > xarray first, then resolve to the actual swap_entry B1.
> 
> Yes. There is an extra lookup.

Ack.

> 
> >
> > > There are some concerns with this design that I outlined in one of my
> > > previous emails, such as having separate swap counting implementation
> > > in different swap backends, which is a maintenance burden and
> > > error-prone.
> >
> > I agree that allocating the swap ID and maintaining the free swap ID
> > would be some extra complexity if we are not reusing the existing swap
> > count code path.
> >
> > My other concern would be the swap ID xarray indirection is always there
> > regardless if you need to use the indirection or not.
> 
> I think the idea is that this design is more minimal than the proposed
> swap_desc, so we can have it behind a config option and remove the
> indirection layer if it is not configured.
> However, I am not yet sure if this would be straightforward. I need to
> give this more thought.

I have given it some thought. I am still concerned about this always on
redirect layer in terms of memory usage and lookup overhead.

Chris