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

From: Yosry Ahmed <yosryahmed@google.com>
To: Chris Li <chrisl@kernel.org>
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: Tue, 28 Mar 2023 15:02:43 -0700	[thread overview]
Message-ID: <CAJD7tkauE=+rWPgLRL0h-D3UVo2Zn23eh1xnnexAK_N30LhmOg@mail.gmail.com> (raw)
In-Reply-To: <ZCNjsopNzd9IU7TX@google.com>

On Tue, Mar 28, 2023 at 3:01 PM Chris Li <chrisl@kernel.org> wrote:
>
> On Tue, Mar 28, 2023 at 02:44:27PM -0700, Yosry Ahmed wrote:
> > > Your swap descriptor will be using one swp_entry_t, which get from the PTE
> > > to lookup, right? That is the swap entry I am talking about. You just
> > > substitute zswap swap entry with the swap descriptor swap entry.
> > > You still need to allocate from the free swap entry space at least once.
> >
> > Oh, you mean the swap ID space. We just need to find an unused ID, we
> > can simply use an allocating xarray
> > (https://docs.kernel.org/core-api/xarray.html#allocating-xarrays).
> > This is simpler than keeping track of swap slots in a swapfile.
>
> Ah I see. That makes sense. Thanks for explaining it to me.
>
> The real block swap device will still need to scan the swap_map
> to find an empty space to write the page.

Yes, exactly. It's a tradeoff, and I think as long as whatever we
decide can be considered as a step towards the right direction it
should be fine.

>
> Chris
>
>