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!
