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

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Jun 7, 2024 at 12:11 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 06.06.24 23:53, Barry Song wrote:
> > On Fri, Jun 7, 2024 at 9:37 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> >>
> >> On Thu, Jun 6, 2024 at 2:30 PM Barry Song <21cnbao@gmail.com> wrote:
> >>>
> >>> On Fri, Jun 7, 2024 at 9:17 AM David Hildenbrand <david@redhat.com> wrote:
> >>>>
> >>>> On 06.06.24 22:31, Yosry Ahmed wrote:
> >>>>> On Thu, Jun 6, 2024 at 1:22 PM David Hildenbrand <david@redhat.com> wrote:
> >>>>>>
> >>>>>> On 06.06.24 20:48, Yosry Ahmed wrote:
> >>>>>>> With ongoing work to support large folio swapin, it is important to make
> >>>>>>> sure we do not pass large folios to zswap_load() without implementing
> >>>>>>> proper support.
> >>>>>>>
> >>>>>>> For example, if a swapin fault observes that contiguous PTEs are
> >>>>>>> pointing to contiguous swap entries and tries to swap them in as a large
> >>>>>>> folio, swap_read_folio() will pass in a large folio to zswap_load(), but
> >>>>>>> zswap_load() will only effectively load the first page in the folio. If
> >>>>>>> the first page is not in zswap, the folio will be read from disk, even
> >>>>>>> though other pages may be in zswap.
> >>>>>>>
> >>>>>>> In both cases, this will lead to silent data corruption.
> >>>>>>>
> >>>>>>> Proper large folio swapin support needs to go into zswap before zswap
> >>>>>>> can be enabled in a system that supports large folio swapin.
> >>>>>>>
> >>>>>>> Looking at callers of swap_read_folio(), it seems like they are either
> >>>>>>> allocated from __read_swap_cache_async() or do_swap_page() in the
> >>>>>>> SWP_SYNCHRONOUS_IO path. Both of which allocate order-0 folios, so we
> >>>>>>> are fine for now.
> >>>>>>>
> >>>>>>> Add a VM_BUG_ON() in zswap_load() to make sure that we detect changes in
> >>>>>>> the order of those allocations without proper handling of zswap.
> >>>>>>>
> >>>>>>> Alternatively, swap_read_folio() (or its callers) can be updated to have
> >>>>>>> a fallback mechanism that splits large folios or reads subpages
> >>>>>>> separately. Similar logic may be needed anyway in case part of a large
> >>>>>>> folio is already in the swapcache and the rest of it is swapped out.
> >>>>>>>
> >>>>>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >>>>>>> ---
> >>>>>>>
> >>>>>>> Sorry for the long CC list, I just found myself repeatedly looking at
> >>>>>>> new series that add swap support for mTHPs / large folios, making sure
> >>>>>>> they do not break with zswap or make incorrect assumptions. This debug
> >>>>>>> check should give us some peace of mind. Hopefully this patch will also
> >>>>>>> raise awareness among people who are working on this.
> >>>>>>>
> >>>>>>> ---
> >>>>>>>     mm/zswap.c | 3 +++
> >>>>>>>     1 file changed, 3 insertions(+)
> >>>>>>>
> >>>>>>> diff --git a/mm/zswap.c b/mm/zswap.c
> >>>>>>> index b9b35ef86d9be..6007252429bb2 100644
> >>>>>>> --- a/mm/zswap.c
> >>>>>>> +++ b/mm/zswap.c
> >>>>>>> @@ -1577,6 +1577,9 @@ bool zswap_load(struct folio *folio)
> >>>>>>>         if (!entry)
> >>>>>>>                 return false;
> >>>>>>>
> >>>>>>> +     /* Zswap loads do not handle large folio swapins correctly yet */
> >>>>>>> +     VM_BUG_ON(folio_test_large(folio));
> >>>>>>> +
> >>>>>>
> >>>>>> There is no way we could have a WARN_ON_ONCE() and recover, right?
> >>>>>
> >>>>> Not without making more fundamental changes to the surrounding swap
> >>>>> code. Currently zswap_load() returns either true (folio was loaded
> >>>>> from zswap) or false (folio is not in zswap).
> >>>>>
> >>>>> To handle this correctly zswap_load() would need to tell
> >>>>> swap_read_folio() which subpages are in zswap and have been loaded,
> >>>>> and then swap_read_folio() would need to read the remaining subpages
> >>>>> from disk. This of course assumes that the caller of swap_read_folio()
> >>>>> made sure that the entire folio is swapped out and protected against
> >>>>> races with other swapins.
> >>>>>
> >>>>> Also, because swap_read_folio() cannot split the folio itself, other
> >>>>> swap_read_folio_*() functions that are called from it should be
> >>>>> updated to handle swapping in tail subpages, which may be questionable
> >>>>> in its own right.
> >>>>>
> >>>>> An alternative would be that zswap_load() (or a separate interface)
> >>>>> could tell swap_read_folio() that the folio is partially in zswap,
> >>>>> then we can just bail and tell the caller that it cannot read the
> >>>>> large folio and that it should be split.
> >>>>>
> >>>>> There may be other options as well, but the bottom line is that it is
> >>>>> possible, but probably not something that we want to do right now.
> >>>>>
> >>>>> A stronger protection method would be to introduce a config option or
> >>>>> boot parameter for large folio swapin, and then make CONFIG_ZSWAP
> >>>>> depend on it being disabled, or have zswap check it at boot and refuse
> >>>>> to be enabled if it is on.
> >>>>
> >>>> Right, sounds like the VM_BUG_ON() really is not that easily avoidable.
> >>>>
> >>>> I was wondering, if we could WARN_ON_ONCE and make the swap code detect
> >>>> this like a read-error from disk.
> >>>>
> >>>> I think do_swap_page() detects that by checking if the folio is not
> >>>> uptodate:
> >>>>
> >>>> if (unlikely(!folio_test_uptodate(folio))) {
> >>>>          ret = VM_FAULT_SIGBUS;
> >>>>          goto out_nomap;
> >>>> }
> >>>>
> >>>> So maybe WARN_ON_ONCE() + triggering that might be a bit nicer to the
> >>>> system (but the app would crash either way, there is no way around it).
> >>>>
> >>>
> >>> I'd rather fallback to small folios swapin instead crashing apps till we fix
> >>> the large folio swapin in zswap :-)
> >>
> >> I think David is referring to catching the buggy cases that do not
> >> properly fallback to small folios with zswap, not as an alternative to
> >> the fallback. This is at least what I had in mind with the patch.
> >
> > Cool. Thanks for the clarification. I'm fine with keeping the fallback,
> > whether it's the current VM_BUG_ON or David's recommended
> > SIGBUS.
> >
> > Currently, mainline doesn't support large folios swap-in, so I see
> > your patch as a helpful warning for those attempting large folio
> > swap-in to avoid making mistakes like loading large folios from
> > zswap.
> >
> > In fact, I spent a week trying to figure out why my app was crashing
> > before I added 'if (zswap_is_enabled()) goto fallback'. If your patch
> > had come earlier, it would have saved me that week of work :-)
> >
> > To me, a direct crash seems like a more straightforward way to
> > prompt people to fallback when attempting large folio swap-ins.
> > So, I am slightly in favor of your current patch.
>
> BUG_ON and friends are frowned-upon, in particular in scenarios where we
> can recover or that are so unexpected that they can be found during
> early testing.
>
> 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.

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.

Barry, I suspect your is_zswap_enabled() check is deficient for
similar reasons, zswap could have been enabled before then became
disabled.