On 2024/1/25 17:26, Yosry Ahmed wrote: > On Thu, Jan 25, 2024 at 1:22 AM Chengming Zhou > <zhouchengming@xxxxxxxxxxxxx> wrote: >> >> On 2024/1/25 17:03, Yosry Ahmed wrote: >>>>>>>> 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. >>>>> >>>>> Why not just call zswap_lru_putback() before we unlock the folio? >>>> >>>> When early return because __read_swap_cache_async() return NULL or !folio_was_allocated, >>>> we don't have a locked folio yet. The entry maybe invalidated and freed concurrently. >>> >>> Oh, that path, right. >>> >>> If we don't isolate the entry straightaway, concurrent reclaimers will >>> see the same entry, call __read_swap_cache_async(), find the folio >>> already in the swapcache and stop shrinking. This is because usually >>> this means we are racing with swapin and hitting the warmer part of >>> the zswap LRU. >>> >>> I am not sure if this would matter in practice, maybe Nhat knows >>> better. Perhaps we can rotate the entry in the LRU before calling >>> __read_swap_cache_async() to minimize the chances of such a race? Or >>> we can serialize the calls to __read_swap_cache_async() but this may >>> be an overkill. >> >> Also, not sure, rotate the entry maybe good IMHO since we will zswap_lru_del() >> once we checked the invalidate race. > > Not sure what you mean. If we rotate first, we won't have anything to > do in the failure case (if the folio is not locked). We will have to > do zswap_lru_del() if actually writeback, yes, but in this case no > synchronization is needed because the folio is locked, right? Right, sorry for my confusing expression. We rotate first in lru lock, and only zswap_lru_del() later if actually writeback. What I want to mean is that the possibility of seeing the entry on lru list by another reclaimer is very low, since we rotate and the timing between __read_swap_cache_async() and zswap_lru_del() should be short.
