Re: [PATCH] mm: zswap: add VM_BUG_ON() if large folio swapin is attempted

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

On Sat, Jun 8, 2024 at 7:17 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> [..]
> > > One problem is that even if zswap was never enabled, the warning will
> > > be emitted just if CONFIG_ZSWAP is on. Perhaps we need a variable or
> > > static key if zswap was "ever" enabled.
> > >
> > > Barry, I suspect your is_zswap_enabled() check is deficient for
> > > similar reasons, zswap could have been enabled before then became
> > > disabled.
> >
> > I don't understand this. if zswap was enabled before but is disabled when
> > I am loading data, will I get corrupted data before zswap was once enabled?
> > If not, it seems nothing important.
>
> If zswap was enabled and then disabled, some pages may still be in
> zswap. We do not load the pages from zswap when it is disabled, we
> just stop storing new pages.
>
> So if you just rely in checking whether zswap is enabled at swapin
> time to decide whether to use large folios, you may end up with a
> situation where zswap is disabled, yet parts of the large folio you
> are trying to swapin (or all of it) is in zswap.
>
> This is why I think we'll need to track whether zswap was ever enabled
> instead (or if a page was ever stored).

Thanks! It doesn't seem good. Do we have a simple way to clean zswap
when it is disabled? seems not easy? Just like we do swapoff, or disable
cache, we ensure they are clean - this is a real "disable".

Thanks
Barry