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

From: Chris Li <chrisl@kernel.org>
To: Rik van Riel <riel@surriel.com>
Cc: Yosry Ahmed <yosryahmed@google.com>,
	Minchan Kim <minchan@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	lsf-pc@lists.linux-foundation.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>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Thu, 2 Mar 2023 16:01:23 -0800	[thread overview]
Message-ID: <ZAE40/qej+2DOXoh@google.com> (raw)
In-Reply-To: <d30e9334a3675813debec03a7525f63d875ecd85.camel@surriel.com>

On Thu, Mar 02, 2023 at 05:36:20PM -0500, Rik van Riel wrote:
> > The questions, do we have this indirection layer apply to all swap
> > entries?
> > 
> I believe we should have a system that tracks every swap entry
> the same, data structure wise. Otherwise we will have two sets
> of code in the kernel, and it will be too easy to get corner
> cases wrong.

It is all about trade offs.  The original proposal is adding 20-30
bytes per swapped out page, compared to existing swap devices.
That is why I want to trade some code complexity for less memory
usage. I will mind less if indirect overhead is much smaller,
for the user who doesn't use it.

> > > That might be a net reduction in the code over what we have today,
> > > because it gets rid of some ugly corner cases.
> > 
> > Great.
> 
> ... but that won't happen if the indirection layer only applies
> to some swap devices, because we will still need to keep around
> the crazy code to deal with the swap devices that don't have it.

I understand your concern.

It is important to have good abstraction between the different
swap devices. I was hoping to good abstraction can isolate out
the difference in different swap devices.

Chris