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

From: Chris Li <chrisl@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Yosry Ahmed <yosryahmed@google.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>,
	Huang Ying <ying.huang@intel.com>, NeilBrown <neilb@suse.de>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Tue, 28 Feb 2023 15:22:20 -0800	[thread overview]
Message-ID: <Y/6MrDGqrMlLS3KK@google.com> (raw)
In-Reply-To: <Y/GmJVEC1zDZpJNR@casper.infradead.org>

Hi Matthew,

On Sun, Feb 19, 2023 at 04:31:33AM +0000, Matthew Wilcox wrote:
> 
> 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

Can you expand a bit on that? I assume you want to see the swap file
behavior more like a normal file system and reuse more of the readpage()
and writepage() path.

> what changes need to be made to swap in order to swap out arbitrary-order
> folios (instead of PMD-sized + PTE-sized).

When the page fault happens, does the whole folios get swapped in or break
into smaller pages?

> 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

I notice the swap_map has one byte per swap entry even the swap is not
used.

> 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?"

All great suggestions and I am very interested in that.

Chris