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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Cheers,

David / dhildenb