On Thu, Jan 25, 2024 at 4:03 PM Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> wrote: > > 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. If you're talking about implementing your solution, I was assuming you were going to send a patch out (and hoping others would chime in in case I missed something). I can take a stab at implementing it if you prefer that, just let me know.
