Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

[Chengming Zhou]

On 2024/1/25 15:53, Yosry Ahmed wrote:
>> Hello,
>>
>> I also thought about this problem for some time, maybe something like below
>> can be changed to fix it? It's likely I missed something, just some thoughts.
>>
>> IMHO, the problem is caused by the different way in which we use zswap entry
>> in the writeback, that should be much like zswap_load().
>>
>> The zswap_load() comes in with the folio locked in swap cache, so it has
>> stable zswap tree to search and lock... But in writeback case, we don't,
>> shrink_memcg_cb() comes in with only a zswap entry with lru list lock held,
>> then release lru lock to get tree lock, which maybe freed already.
>>
>> So we should change here, we read swpentry from entry with lru list lock held,
>> then release lru lock, to try to lock corresponding folio in swap cache,
>> if we success, the following things is much the same like zswap_load().
>> We can get tree lock, to recheck the invalidate race, if no race happened,
>> we can make sure the entry is still right and get refcount of it, then
>> release the tree lock.
> 
> Hmm I think you may be onto something here. Moving the swap cache
> allocation ahead before referencing the tree should give us the same
> guarantees as zswap_load() indeed. We can also consolidate the
> invalidate race checks (right now we have one in shrink_memcg_cb() and
> another one inside zswap_writeback_entry()).
> 
> We will have to be careful about the error handling path to make sure
> we delete the folio from the swap cache only after we know the tree
> won't be referenced anymore. Anyway, I think this can work.
> 
> On a separate note, I think there is a bug in zswap_writeback_entry()
> when we delete a folio from the swap cache. I think we are missing a
> folio_unlock() there.
> 

Hi, want to know if you are preparing the fix patch, I would just wait to
review if you are. Or I can work on it if you are busy with other thing.

Thanks!