On 6/30/25 9:24 AM, Jens Axboe wrote: > On 6/30/25 5:28 AM, Yu Kuai wrote: >> From: Yu Kuai <yukuai3@xxxxxxxxxx> >> >> __xa_cmpxchg() is called with rcu_read_lock(), and it will allocate >> memory if necessary. >> >> Fix the problem by moving rcu_read_lock() after __xa_cmpxchg(), meanwhile, >> it still should be held before xa_unlock(), prevent returned page to be >> freed by concurrent discard. > > The rcu locking in there is a bit of a mess, imho. What _exactly_ is the > rcu read side locking protecting? Is it only needed around the lookup > and insert? We even hold it over the kmap and copy, which seems very > heavy handed. Gah it's holding the page alive too. Can't we just grab a ref to the page when inserting it, and drop that at free time? It would be a lot better to have only the lookup be RCU protected, having the full copies under it seems kind of crazy. IOW, I think there's room for some good cleanups here. -- Jens Axboe
