Re: [PATCH v5 01/11] mm, swap: add virtual swap device infrastructure

[Rik van Riel <riel@surriel.com>]

On Thu, 2026-09-24 at 15:17 -1000, Chris Li wrote:
> On Thu, Sep 24, 2026 at 8:13 AM Rik van Riel <riel@surriel.com>
> wrote:
> > 
> > On Tue, 2026-09-22 at 23:18 -1000, Chris Li wrote:
> > > On Fri, Sep 18, 2026 at 1:03 PM Nhat Pham <nphamcs@gmail.com>
> > > wrote:
> > > > 
> > > > 
> > > > @@ -482,6 +491,11 @@ void swap_read_folio(struct swap_io_ctx
> > > > *ctx,
> > > > struct folio *folio)
> > > >         if (zswap_load(folio) != -ENOENT)
> > > >                 goto finish;
> > > > 
> > > > +       if (unlikely(swap_is_vswap(sis))) {
> > > 
> > > There are too many swap_is_vswap() in this series. It fragments
> > > the
> > > code path, making things harder to reason about. Esepcially
> > > around
> > > locks. I count 22 in this patch alone.
> > > 
> > > I consider this the biggest drawback of this series. This
> > > fragmentation of the code path.
> > 
> > Could we avoid that by simply always having everything
> > go through the vswap abstraction layer?
> 
> Yes, but it will pay the price for going through the unnecessary
> redirection layer always. It incurs performance and meta data
> overhead. That was one of my previous objections to the earlier vswap
> version. Sorry I enjoy micro optimization too much, that is both my
> strength and weakness.

We really only have two cases here, don't we?

Either the data is in the swap cache, and
the vswap layer can directly look up the
swap cache page.

Or the data is in some slower back-end,
and doing a second lookup is not going to
introduce noticeable overhead.

If the micro optimization come at the cost
of less flexibility, and harder to maintain
code, are they really worth it?

> 
> > Handling the details of what's behind the vswap would
> > be handled one layer down.
> > 
> > Chris, do you think that would be cleaner?
> 
> If it can wrap below the swap_ops, it is just priviate inernal dedail
> of implementing its own indirections. That would be much cleaner.
> That
> is what I am trying to pitch to Nhat in prevoius email but does not
> have following actions.
> 
Can you sketch out the details of what you
think that should look like?

What would the data structures look like?

What operations should there be at the
abstraction layer, and at the swap layers
below?

How should migration of data from one swap
backend to another be handled?

At this point the best way to make progress
would be to hash out those details.

If you are unhappy with what you have seen
from others, what does your ideal design
look like?

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