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

[Minchan Kim <minchan@kernel.org>]

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. ;-)

> 
> I am intending to spend more time looking further into this, but other
> things keep popping up :)

Same with me. Thanks for looking it, Yosry!