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

[Yosry Ahmed]

[..]
> > > >
> > > > How about something like the following (untested), it is the minimal
> > > > recovery we can do but should work for a lot of cases, and does
> > > > nothing beyond a warning if we can swapin the large folio from disk:
> > > >
> > > > diff --git a/mm/page_io.c b/mm/page_io.c
> > > > index f1a9cfab6e748..8f441dd8e109f 100644
> > > > --- a/mm/page_io.c
> > > > +++ b/mm/page_io.c
> > > > @@ -517,7 +517,6 @@ void swap_read_folio(struct folio *folio, struct
> > > > swap_iocb **plug)
> > > >          delayacct_swapin_start();
> > > >
> > > >          if (zswap_load(folio)) {
> > > > -               folio_mark_uptodate(folio);
> > > >                  folio_unlock(folio);
> > > >          } else if (data_race(sis->flags & SWP_FS_OPS)) {
> > > >                  swap_read_folio_fs(folio, plug);
> > > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > > index 6007252429bb2..cc04db6bb217e 100644
> > > > --- a/mm/zswap.c
> > > > +++ b/mm/zswap.c
> > > > @@ -1557,6 +1557,22 @@ bool zswap_load(struct folio *folio)
> > > >
> > > >          VM_WARN_ON_ONCE(!folio_test_locked(folio));
> > > >
> > > > +       /*
> > > > +        * Large folios should not be swapped in while zswap is being used, as
> > > > +        * they are not properly handled.
> > > > +        *
> > > > +        * If any of the subpages are in zswap, reading from disk would result
> > > > +        * in data corruption, so return true without marking the folio uptodate
> > > > +        * so that an IO error is emitted (e.g. do_swap_page() will sigfault).
> > > > +        *
> > > > +        * Otherwise, return false and read the folio from disk.
> > > > +        */
> > > > +       if (WARN_ON_ONCE(folio_test_large(folio))) {
> > > > +               if (xa_find(tree, &offset, offset +
> > > > folio_nr_pages(folio) - 1, 0))
> > > > +                       return true;
> > > > +               return false;
> > > > +       }
> > > > +
> > > >          /*
> > > >           * When reading into the swapcache, invalidate our entry. The
> > > >           * swapcache can be the authoritative owner of the page and
> > > > @@ -1593,7 +1609,7 @@ bool zswap_load(struct folio *folio)
> > > >                  zswap_entry_free(entry);
> > > >                  folio_mark_dirty(folio);
> > > >          }
> > > > -
> > > > +       folio_mark_uptodate(folio);
> > > >          return true;
> > > >   }
> > > >
> > > > 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.
> > >
> > > We should use WARN_ON_ONCE() only for things that cannot happen. So if
> > > there are cases where this could be triggered today, it would be
> > > problematic -- especially if it can be triggered from unprivileged user
> > > space. But if we're concerned of other code messing up our invariant in
> > > the future (e.g., enabling large folios without taking proper care about
> > > zswap etc), we're good to add it.
> >
> > Right now I can't see any paths allocating large folios for swapin, so
> > I think it cannot happen. Once someone tries adding it, the warning
> > will fire if CONFIG_ZSWAP is used, even if zswap is disabled.
> > At this point we will have several options:
>
> Here is my take on this:
>
> > - Make large folios swapin depend on !CONFIG_ZSWAP for now.
>
> I think a WARON or BUG_ON is better. I would need to revert this
> change when I am working on 3).  It is a make up rule, not a real
> dependency any way.

I am intending to send a new version with WARN_ON_ONCE() and some
attempt to recover.

It is not a rule, it is just that we don't have the support for it today.

>
> > - Keep track if zswap was ever enabled and make the warning
> > conditional on it. We should also always fallback to order-0 if zswap
> > was ever enabled.
>
> IMHO, falling back to order-0 inside zswap is not desired because it
> complicates the zswap code. We should not pass large folio to zswap if
> zswap is not ready to handle large folio. The core swap already has
> the fall back to order-0. If we get to 3), then this fall back in
> zswap needs to be removed. It is a transitional thing then maybe not
> introduce it in the first place.

We cannot split the folio inside zswap. What I meant is that the
swapin code should fallback to order-0 if zswap is being used, to
avoid passing large folios to zswap.

>
> > - Properly handle large folio swapin with zswap.
> That obviously is ideal.
>
> Chris