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

From: Rik van Riel <riel@surriel.com>
To: Chris Li <chrisl@kernel.org>
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, 02 Mar 2023 17:36:20 -0500	[thread overview]
Message-ID: <d30e9334a3675813debec03a7525f63d875ecd85.camel@surriel.com> (raw)
In-Reply-To: <ZAEYTBS0ipKOa6+w@google.com>

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On Thu, 2023-03-02 at 13:42 -0800, Chris Li wrote:
> On Thu, Mar 02, 2023 at 01:23:14PM -0500, Rik van Riel wrote:
> > 
> > 
> > One possible implementation might be to have swap page table
> > entries
> > point to a swap address in this indirection layer, and the
> > indirection
> > layer can be an xarray containing the actual swap entries
> > specifying
> > at which position in which swap device the data can be found.
> 
> 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.

> My small tweak is to limit the indirection layer only to non leaf
> swap devices. Then it is actually very close to what I am proposing.
> Just your "indirection layer" is my "special swap device".
> 
> Again, "special swap device" is a very bad name, let's name it
> something
> more useful.
> 
> > 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.

-- 
All Rights Reversed.

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