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

From: Matthew Wilcox <willy@infradead.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: 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>,
	Huang Ying <ying.huang@intel.com>, NeilBrown <neilb@suse.de>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Sun, 19 Feb 2023 04:31:33 +0000	[thread overview]
Message-ID: <Y/GmJVEC1zDZpJNR@casper.infradead.org> (raw)
In-Reply-To: <CAJD7tkbCnXJ95Qow_aOjNX6NOMU5ovMSHRC+95U4wtW6cM+puw@mail.gmail.com>

On Sat, Feb 18, 2023 at 02:38:40PM -0800, Yosry Ahmed wrote:
> Hello everyone,
> 
> I would like to propose a topic for the upcoming LSF/MM/BPF in May
> 2023 about swap & zswap (hope I am not too late).

Submissions are due March 1st, I believe, so not too late.

> ==================== Bottom Line ====================
> It would be nice to discuss the potential here and the tradeoffs. I
> know that other folks using zswap (or interested in using it) may find
> this very useful. I am sure I am missing some context on why things
> are the way they are, and perhaps some obvious holes in my story.
> Looking forward to discussing this with anyone interested :)
> 
> I think Johannes may be interested in attending this discussion, since
> a lot of ideas here are inspired by discussions I had with him :)

I think an overhaul of the swap code is long overdue.  I appreciate
you're very much focused on zswap, but there are many other problems.
For example, swap does not work on zoned devices.  Swap readahead is
generally physical (ie optimised for spinning discs) rather than logical
(more appropriate for SSDs).  Swap's management of free space is crude
compared to real filesystems.  The way that swap bypasses the filesystem
when writing to swap files is awful.  I haven't even started to look at
what changes need to be made to swap in order to swap out arbitrary-order
folios (instead of PMD-sized + PTE-sized).

I'm probably not a great person to participate in the design of a
replacement system.  I don't know nearly enough about anonymous memory.
I'd be sitting in the back shouting unhelpful things like, "Can't you
see an anon_vma is the exact same thing as an inode?"  and "Why don't
we steal the block allocation functions from XFS?"  and "Why do tmpfs
pages have to move to the swap cache; can't we just leave them in the
page cache and pass them to the swap code directly?"

Maybe Neil Brown or Huang Ying would be good participants, although
I don't recall seeing either of them at an LSFMM recently.