On 2024/1/26 08:05, Yosry Ahmed wrote: > 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). Ok, I will prepare a patch to send out for further discussion. > > I can take a stab at implementing it if you prefer that, just let me know.
