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 6:58 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Fri, Jun 7, 2024 at 11:52 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > >> I have no strong opinion on this one, but likely a VM_WARN_ON would also
> > >> be sufficient to find such issues early during testing. No need to crash
> > >> the machine.
> > >
> > > I thought VM_BUG_ON() was less frowned-upon than BUG_ON(), but after
> > > some digging I found your patches to checkpatch and Linus clearly
> > > stating that it isn't.
> >
> > :) yes.
> >
> > VM_BUG_ON is not particularly helpful IMHO. If you want something to be
> > found early during testing, VM_WARN_ON is good enough.
> >
> > Ever since Fedora stopped enabling CONFIG_DEBUG_VM, VM_* friends are
> > primarily reported during early/development testing only. But maybe some
> > distro out there still sets it.
> >
> > >
> > > 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:
> - Make large folios swapin depend on !CONFIG_ZSWAP for now.

It appears quite problematic. We lack control over whether the kernel build
will enable CONFIG_ZSWAP, particularly when aiming for a common
defconfig across all platforms to streamline configurations. For instance,
in the case of ARM, this was once a significant goal.

Simply trigger a single WARN or BUG if an attempt is made to load
large folios in zswap_load, while ensuring that zswap_is_enabled()
remains unaffected. In the mainline code, large folio swap-in support
is absent, so this warning is intended for debugging purposes and
targets a very small audience—perhaps fewer than five individuals
worldwide. Real users won’t encounter this warning, as it remains
hidden from their view.

> - 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.
> - Properly handle large folio swapin with zswap.
>
> Does this sound reasonable to you?

Thanks
Barry