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

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Aug 5, 2025 at 3:47 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.08.25 02:29, SeongJae Park wrote:
> > When zswap writeback is enabled and it fails compressing a given page,
> > the page is swapped out to the backing swap device.  This behavior
> > breaks the zswap's writeback LRU order, and hence users can experience
> > unexpected latency spikes.  If the page is compressed without failure,
> > but results in a size of PAGE_SIZE, the LRU order is kept, but the
> > decompression overhead for loading the page back on the later access is
> > unnecessary.
> >
> > Keep the LRU order and optimize unnecessary decompression overheads in
> > the cases, by storing the original content in zpool as-is.
>
> Does this have any effect on the movability of the given page? IOW, does
> page migration etc. still work when we store an ordinary page of an
> shmem/anon folio here?

Good question. This depends on the backend allocator of zswap, but the
only backend allocator remaining (zsmalloc) does implement page
migration.

It's why we insisted on using zpool/zsmalloc to handle the
incompressibility case as well:

https://lore.kernel.org/all/CAKEwX=NC65XCkmX1YzivEJtPc+sEJ3pLHUsYhF60QJnk_OtpVw@mail.gmail.com/

>
> --
> Cheers,
>
> David / dhildenb
>