Re: [PATCH 2/5] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks

[Yosry Ahmed]

On Thu, Nov 3, 2022 at 2:43 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> On Thu, Nov 03, 2022 at 01:46:56PM -0700, Yosry Ahmed wrote:
> > On Thu, Nov 3, 2022 at 1:37 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Nov 03, 2022 at 11:10:47AM -0700, Yosry Ahmed wrote:
> > > < snip >
> > >
> > > > > > > > I am also worry about that LRU stuff should be part of allocator
> > > > > > > > instead of higher level.
> > > > > > >
> > > > > > > I'm sorry, but that's not a reasonable objection.
> > > > > > >
> > > > > > > These patches implement a core feature of being a zswap backend, using
> > > > > > > standard LRU and locking techniques established by the other backends.
> > > > > > >
> > > > > > > I don't disagree that it would nicer if zswap had a strong abstraction
> > > > > > > for backend pages and a generalized LRU. But that is major surgery on
> > > > > > > a codebase of over 6,500 lines. It's not a reasonable ask to change
> > > > > > > all that first before implementing a basic feature that's useful now.
> > > > > >
> > > > > > With same logic, folks added the LRU logic into their allocators
> > > > > > without the effort considering moving the LRU into upper layer.
> > > > > >
> > > > > > And then trend is still going on since I have seen multiple times
> > > > > > people are trying to add more allocators. So if it's not a reasonable
> > > > > > ask to consier, we couldn't stop the trend in the end.
> > > > >
> > > > > So there is actually an ongoing effort to do that. Yosry and I have
> > > > > spent quite some time on coming up with an LRU design that's
> > > > > independent from compression policy over email and at Plumbers.
> > > > >
> > > > > My concern is more about the order of doing things:
> > > > >
> > > > > 1. The missing writeback support is a gaping hole in zsmalloc, which
> > > > >    affects production systems. A generalized LRU list is a good idea,
> > > > >    but it's a huge task that from a user pov really is not
> > > > >    critical. Even from a kernel dev / maintainer POV, there are bigger
> > > > >    fish to fry in the zswap code base and the backends than this.
> > > > >
> > > > > 2. Refactoring existing functionality is much easier than writing
> > > > >    generalized code that simultaneously enables new behavior. zsmalloc
> > > > >    is the most complex of our backends. To make its LRU writeback work
> > > > >    we had to patch zswap's ->map ordering to accomodate it, e.g. Such
> > > > >    tricky changes are easier to make and test incrementally.
> > > > >
> > > > >    The generalized LRU project will hugely benefit from already having
> > > > >    a proven writeback implementation in zsmalloc, because then all the
> > > > >    requirements in zswap and zsmalloc will be in black and white.
> > > > >
> > > > > > > I get that your main interest is zram, and so this feature isn't of
> > > > > > > interest to you. But zram isn't the only user, nor is it the primary
> > > > > >
> > > > > > I am interest to the feature but my interest is more of general swap
> > > > > > layer to manage the LRU so that it could support any hierarchy among
> > > > > > swap devices, not only zswap.
> > > > >
> > > > > I think we're on the same page about the longer term goals.
> > > > >
> > > >
> > > > Yeah. As Johannes said, I was also recently looking into this. This
> > > > can also help solve other problems than consolidating implementations.
> > > > Currently if zswap rejects a page, it goes into swap, which is
> > > > more-or-less a violation of page LRUs since hotter pages that are more
> > > > recently reclaimed end up in swap (slow), while colder pages that were
> > > > reclaimed before are in zswap. Having a separate layer managing the
> > > > LRU of swap pages can also make sure this doesn't happen.
> > >
> > > True.
> > >
> > > >
> > > > More broadly, making zswap a separate layer from swap enables other
> > > > improvements such as using zswap regardless of the presence of a
> > > > backend swapfile and not consuming space in swapfiles if a page is in
> > > > zswap. Of course, this is a much larger surgery.
> > >
> > > If we could decouple the LRU writeback from zswap and supports
> > > compression without backing swapfile, sounds like becoming more of
> > > zram. ;-)
> >
> > That's a little bit grey. Maybe we can consolidate them one day :)
> >
> > We have been using zswap without swapfile at Google for a while, this
> > gives us the ability to reject pages that do not compress well enough
> > for us, which I suspect zram would not support given that it is
> > designed to be the final destination of the page. Also, having the
>
> zRAM could do with little change but at current implmentation, it will
> print swapout failure message(it's not a big deal since we could
> suppress) but I have thought rather than that, we needs to move the
> page unevictable LRU list with marking the page CoW to catch a time
> to move the page into evictable LRU list o provide second chance to
> be compressed. Just off-topic.

Right. We do something similar-ish today. However, this does not work
though for zswap if there is a backing swapfile, as the page needs to
still be evictable to the swapfile. A decoupled LRU can also manage
this appropriately.

>
> > same configuration and code running on machines whether or not they
> > have a swapfile is nice, otherwise one would need to use zram if there
> > is no swapfile and switch to zswap if there is one.