Re: [PATCH 0/3] mm: zswap: trivial folio conversions

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, May 24, 2024 at 12:53 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Thu, May 23, 2024 at 8:59 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, May 24, 2024 at 03:38:15AM +0000, Yosry Ahmed wrote:
> > > Some trivial folio conversions in zswap code.
> >
> > The three patches themselves look good.
> >
> > > The mean reason I included a cover letter is that I wanted to get
> > > feedback on what other trivial conversions can/should be done in
> > > mm/zswap.c (keeping in mind that only order-0 folios are supported
> > > anyway).  These are the things I came across while searching for 'page'
> > > in mm/zswap.c, and chose not to do anything about for now:
> >
> > I think there's a deeper question to answer before answering these
> > questions, which is what we intend to do with large folios and zswap in
> > the future.  Do we intend to split them?  Compress them as a large
> > folio?  Compress each page in a large folio separately?  I can see an
> > argument for choices 2 and 3, but I think choice 1 is going to be
> > increasingly untenable.
>
> Yeah I was kinda getting the small things out of the way so that zswap
> is fully folio-ized, before we think about large folios. I haven't
> given it a lot of thought, but here's what I have in mind.
>
> Right now, I think most configs enable zswap will disable
> CONFIG_THP_SWAP (otherwise all THPs will go straight to disk), so
> let's assume that today we are splitting large folios before they go
> to zswap (i.e. choice 1).
>
> What we do next depends on how the core swap intends to deal with
> large folios. My understanding based on recent developments is that we
> intend to swapout large folios as a whole, but I saw some discussions
> about splitting all large folios before swapping them out, or leaving
> them whole but swapping them out in order-0 chunks.
>
> I assume the rationale is that there is little benefit to keeping the
> folios whole because they will most likely be freed soon anyway, but I
> understand not wanting to spend time on splitting them, so swapping
> them out in order-0 chunks makes some sense to me. It also dodges the
> whole fragmentation issue.
>
> If we do either of these things in the core swap code, then I think
> zswap doesn't need to do anything to support large folios. If not,
> then we need to make a choice between 2 (compress large folios) &
> choice 3 (compress each page separately) as you mentioned.
>
> Compressing large folios as a whole means that we need to decompress
> them as a whole to read a single page, which I think could be very
> inefficient in some cases or force us to swapin large folios. Unless
> of course we end up in a world where we mostly swapin the same large
> folios that we swapped out. Although there can be additional
> compression savings from compressing large folios as a whole.
>
> Hence, I think choice 3 is the most reasonable one, at least for the
> short-term. I also think this is what zram does, but I haven't
> checked. Even if we all agree on this, there are still questions that
> we need to answer. For example, do we allocate zswap_entry's for each
> order-0 chunk right away, or do we allocate a single zswap_entry for
> the entire folio, and then "split" it during swapin if we only need to
> read part of the folio?
>
> Wondering what others think here.

More thoughts that came to mind here:

- Whether we go with choice 2 or 3, we may face a latency issue. Zswap
compression happens synchronously in the context of reclaim, so if we
start handling large folios in zswap, it may be more efficient to do
it asynchronously like swap to disk.

- Supporting compression of large folios depends on zsmalloc (and
maybe other allocators) supporting it. There have been patches from
Barry to add this support to some extent, but I didn't take a close
look at those.

Adding other folks from the mTHP swap discussions here in case they
have other thoughts about zswap.