Re: [PATCH 2/3] mm/swap: use swap_ops to register swap device's methods

[Baoquan He <bhe@redhat.com>]

On 03/02/26 at 01:21pm, Nhat Pham wrote:
> On Mon, Mar 2, 2026 at 2:40 AM Baoquan He <bhe@redhat.com> wrote:
...snip...
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 915bc93964db..af498f9af328 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3625,6 +3625,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> >         /* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */
> >         enable_swap_info(si);
> >
> > +       probe_swap_fs(si);
> > +
> >         pr_info("Adding %uk swap on %s.  Priority:%d extents:%d across:%lluk %s%s%s%s\n",
> >                 K(si->pages), name->name, si->prio, nr_extents,
> >                 K((unsigned long long)span),
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index a399f7a10830..7ce906249c7a 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1055,7 +1055,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
> >         folio_set_reclaim(folio);
> >
> >         /* start writeback */
> > -       __swap_writepage(folio, NULL);
> > +       if (si->ops && si->ops->write_folio)
> > +               si->ops->write_folio(si, folio, NULL);
> 
> This one seems really weird to me TBH...
> 
> What's the point of a zswap writeback to a backend without writeback
> operation...? We're loading the coldest zswap objects back into
> memory...?
> 
> More generally, do you anticipate a swap backend with no write_folio?
> I can't imagine such backend.
> 
> That said, if they exist, they should be exempted from zswap
> writeback. At the very least, move this check to the beginning of
> zswap_writeback_entry(), right after swap device pinning
> (get_swap_device()):
> 
> 1. If swap device has no writeback op, just bail. Don't even bother
> allocate swap cache, decompression, etc.
> 
> 2. If you anticipate that this case will never happen, add a warn.

Good point. Currently, there's no case where a swap backend without
write_folio. While in the future there could be, assume you have noticed
the swap tier patchset introduced by Youngjun.

Here, the si->ops checking is purely defensive programming. I can add a
warn at the beginnin of zswap_writeback_entry(). If there's any case
involving no read/write_folio, adaption can be made at that time.