Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Swap Abstraction "the pony"

[Jared Hulbert <jaredeh@gmail.com>]

On Thu, Mar 7, 2024 at 9:35 AM Jan Kara <jack@suse.cz> wrote:
>
> Well, but then if you fill in space of a particular order and need to swap
> out a page of that order what do you do? Return ENOSPC prematurely?
>
> Frankly as I'm reading the discussions here, it seems to me you are trying
> to reinvent a lot of things from the filesystem space :) Like block
> allocation with reasonably efficient fragmentation prevention, transparent
> data compression (zswap), hierarchical storage management (i.e., moving
> data between different backing stores), efficient way to get from
> VMA+offset to the place on disk where the content is stored. Sure you still
> don't need a lot of things modern filesystems do like permissions,> directory structure (or even more complex namespacing stuff), all the stuff
> achieving fs consistency after a crash, etc. But still what you need is a
> notable portion of what filesystems do.
>
> So maybe it would be time to implement swap as a proper filesystem? Or even
> better we could think about factoring out these bits out of some existing
> filesystem to share code?

Yes.  Thank you.  I've been struggling to communicate this.

I'm thinking you can just use existing filesystems as a first step
with a modest glue layer.  See the branch of this thread where I'm
babbling on to Chris about this.

"efficient way to get from VMA+offset to place on the disk where
content is stored"
You mean treat swapped pages like they were mmap'ed files and use the
same code paths?  How big of a project is that?  That seems either
deceptively easy or really hard... I've been away too long and was
never really good enough to have a clear vision of the scale.

On the file side we have the page cache, but on the swap side you have
swap cache and zswap. If we reconciled file pages and swap pages you
could have page cache and zpage_cache(?) bringing gains in both
directions.  If the argument is that the swap fault path is a lot
faster, then shouldn't we be talking about fixing the file fault path
anyway?

I'd love to hear the real experts chime in.