Re: [PATCH v2] mm/zswap: store <PAGE_SIZE compression failed page as-is

[Chris Li]

On Wed, Aug 13, 2025 at 1:48 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Wed, Aug 13, 2025 at 12:42:32PM -0700, Shakeel Butt wrote:
> > On Wed, Aug 13, 2025 at 10:07:18AM -0700, Chris Li wrote:
> > >
> > > If you store uncompressed data in the zpool, zpool has metadata
> > > overhead, e.g. allocating the entry->handle for uncompressed pages.
> > > If the page is not compressed, another idea is just skip the zpool,
> > > store it as a page in the zswap entry as page. We can make a union of
> > > entry->handle and entry->incompressble_page. If entry->length ==
> > > PAGE_SIZE, use entry->incompressable_page as a page.
> >
> > The main problem being solved here is to avoid the scenario where the
> > incompressible pages are being rotated in LRUs and zswapped multiple
> > times and wasting CPU on compressing incompressible pages. SJ's approach
> > solves the issue but with some memory overhead (zswap entry). With your
> > suggestion and to solve the mentioned issue, we will need to change some
> > core parts of reclaim (__remove_mapping()), LRU handling (swap cache
> > pages not in LRUs) and refault (putting such pages back in LRU and
> > should it handle read and write faults differently). So, the cons of
> > that approach is more complex code.
>
> What Chris is proposing would also fix that, even for configurations
> without writeback. So I'm not opposed to it.
>
> However, for deployments where writeback *is* enabled, this code is an
> improvement over the status quo. And it's not in conflict with a
> broader fix for !writeback setups, so it's not an either-or scenario.
>
> Specifically for the writeback case, the metadata overhead is not much
> of a concern: we can just write back more zswap tail to make up for
> it; the more important thing is that we can now do so in LRU order.
>
> The premise being that writing an additional cold page from zswap to
> disk to make room for a slightly inefficiently stored warm page is
> better than rejecting and sending the *warm* page to disk instead.
>
> So I agree with you Chris. But also think that's follow-up work for
> somebody who cares about the !writeback case.

Thank you and I agree with your assessment.

Chris