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

[Barry Song]

On Wed, May 29, 2024 at 7:08 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Fri, May 24, 2024 at 4:13 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > On Fri, May 24, 2024 at 12:53 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > >
> > > On Thu, May 23, 2024 at 8:59 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.
>
> We've been discussing this in private as well :)
>
> It doesn't have to be these two extremes right? I'm perfectly happy
> with starting with compressing each subpage separately, but perhaps we
> can consider managing larger folios in bigger chunks (say 64KB). That
> way, on swap-in, we just have to bring a whole chunk in, not the
> entire folio, and still take advantage of compression efficiencies on
> bigger-than-one-page chunks. I'd also check with other filesystems
> that leverage compression, to see what's their unit of compression is.
>
> I believe this is the approach Barry is suggesting for zram:
>
> https://lore.kernel.org/linux-block/20240327214816.31191-2-21cnbao@xxxxxxxxx/T/
>
> Once the zsmalloc infrastructure is there, we can play with this :)
>
> Barry - what's the progress regarding this front?

Thanks for reaching out.

Not too much. It depends on large folios swap-in because we need to swap in
large folios if we compress them as a whole. For example, if we swap out
64KiB but only swap in 4KiB, we still need to decompress the entire 64KiB
but copy only 4KiB.

Recently, we’ve only addressed the large folio swap-in refault cases in
the mm-unstable branch[1].
[1] https://lore.kernel.org/linux-mm/20240529082824.150954-1-21cnbao@xxxxxxxxx/

Currently, swap-in is not allocating large folios in any mm branch.

A major debate is that my original patch[2] started from SYNC_IO case for zRAM
and embedded devices first, while Ying argue we should start from non-SYNC
IO and decide swapin sizes based on read-ahead window but not based on
the original sizes of how folios are swapped out.
[2] https://lore.kernel.org/linux-mm/20240304081348.197341-6-21cnbao@xxxxxxxxx/

So I guess we need more work to get large folios swap-in ready, and it
won't happen
shortly.

>
> >
> > - 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.

Thanks
Barry