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()). Right, if we successfully lock folio in the swap cache, we can get the tree lock and check the invalidate race, only once. > > 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. Yes, we can't reference tree if we early return or after unlocking folio, since the reference of zswap entry can't protect the tree. > > 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. Ah, yes, and folio_put(). > >> >> The main differences between this writeback with zswap_load() is the handling >> of lru entry and the tree lifetime. The whole zswap_load() function has the >> stable reference of zswap tree, but it's not for shrink_memcg_cb() bottom half >> after __swap_writepage() since we unlock the folio after that. So we can't >> reference the tree after that. >> >> This problem is easy to fix, we can zswap_invalidate_entry(tree, entry) early >> in tree lock, since thereafter writeback can't fail. BTW, I think we should >> also zswap_invalidate_entry() early in zswap_load() and only support the >> zswap_exclusive_loads_enabled mode, but that's another topic. > > zswap_invalidate_entry() actually doesn't seem to be using the tree at all. > >> >> The second difference is the handling of lru entry, which is easy that we >> just zswap_lru_del() in tree lock. > > Why do we need zswap_lru_del() at all? We should have already isolated > the entry at that point IIUC. I was thinking how to handle the "zswap_lru_putback()" if not writeback, in which case we can't use the entry actually since we haven't got reference of it. So we can don't isolate at the entry, and only zswap_lru_del() when we are going to writeback actually. Thanks!
