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

[Barry Song <21cnbao@gmail.com>]

On Fri, Mar 8, 2024 at 5:06 AM Jared Hulbert <jaredeh@gmail.com> wrote:
>
> 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.

I don't understand why we need this level of complexity. All we need to know
are the offsets during pageout. After that, the large folio is
destroyed, and all
offsets are stored in page table entries (PTEs) or xa. Swap-in doesn't depend
on a complex file system; it can make its own decision on how to swap-in
based on the values it reads from PTEs.

Swap-in doesn't need to know whether the swapped-out folio was large or not.

>
> 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.

Thanks
Barry