On 25 Oct 2024, at 1:41, Hugh Dickins wrote: > On Thu, 24 Oct 2024, Zi Yan wrote: >> On 24 Oct 2024, at 0:10, Hugh Dickins wrote: >> >>> The new unlocked list_del_init() in deferred_split_scan() is buggy. >>> I gave bad advice, it looks plausible since that's a local on-stack >>> list, but the fact is that it can race with a third party freeing or >>> migrating the preceding folio (properly unqueueing it with refcount 0 >>> while holding split_queue_lock), thereby corrupting the list linkage. >>> >>> The obvious answer would be to take split_queue_lock there: but it has >>> a long history of contention, so I'm reluctant to add to that. Instead, >>> make sure that there is always one safe (raised refcount) folio before, >>> by delaying its folio_put(). (And of course I was wrong to suggest >>> updating split_queue_len without the lock: leave that until the splice.) >> >> I feel like this is not the right approach, since it breaks the existing >> condition of changing folio->_deferred_list, namely taking >> ds_queue->split_queue_lock for serialization. The contention might not be >> as high as you think, since if a folio were split, the split_queue_lock >> needed to be taken during split anyway. So the worse case is the same >> as all folios are split. Do you see significant perf degradation due to >> taking the lock when doing list_del_init()? >> >> I am afraid if we take this route, we might hit hard-to-debug bugs >> in the future when someone touches the code. > > You have a good point: I am adding another element of trickiness > to that already-tricky local-but-not-quite list - which has tripped > us up a few times in the past. > > I do still feel that this solution is right in the spirit of that list; > but I've certainly not done any performance measurement to justify it, > nor would I ever trust my skill to do so. I just tried to solve the > corruptions in what I thought was the best way. > > (To be honest, I found this solution to the corruptions first, and thought > the bug went back to the original implemention: that its put_page() at the > end of the loop was premature all along. It was only when writing the > commit message two days ago, that I came to realize that even put_page() > or folio_put() would be safely using the lock to unqueue: that it is only > this new list_del_init() which is the exception which introduces the bug.) > > Looking at vmstats, I'm coming to believe that the performance advantage > of this way is likely to be in the noise: that mTHPs alone, and the > !partially_mapped case on top, are greatly increasing the split_deferred > stats: and may give rise to renewed complaints of lock contention, with > or without this optimization. > > While I still prefer to stick with what's posted and most tested, I am > giving the locked version a run overnight. Thanks a lot for the reviews > and acks everyone: at present Zi Yan is in the minority preferring a > locked version, but please feel free to change your vote if you wish. Thank you a lot for taking the time to check the locked version. Looking forward to the result. BTW, I am not going to block this patch since it fixes the bug. The tricky part in deferred_list_scan() is always the use of folio->_deferred_list without taking split_queue_lock. I am thinking about use folio_batch to store the out-of-split_queue folios, so that _deferred_list will not be touched when these folios are tried to be split. Basically, 1. loop through split_queue and move folios to a folio_batch until the folio_batch is full; 2. loop through the folio_batch to try to split each folio; 3. move the remaining folios back to split_queue. With this approach, split_queue_lock might be taken more if there are more than 31 (folio_batch max size) folios on split_queue and split_queue_lock will be held longer in step 3, since the remaining folios need to be added back to split_queue one by one instead of a single list splice. Let me know your thoughts. I can look into this if this approach sounds promising. Thanks. Best Regards, Yan, Zi
