On 24/07/2020 01:24, Jens Axboe wrote: > On 7/23/20 4:16 PM, Jens Axboe wrote: >> On 7/23/20 12:12 PM, Pavel Begunkov wrote: >>> poll_add can have req->work initialised, which will be overwritten in >>> __io_arm_poll_handler() because of the union. Luckily, hash_node is >>> zeroed in the end, so the damage is limited to lost put for work.creds, >>> and probably corrupted work.list. >>> >>> That's the easiest and really dirty fix, which rearranges members in the >>> union, arm_poll*() modifies and zeroes only work.files and work.mm, >>> which are never taken for poll add. >>> note: io_kiocb is exactly 4 cachelines now. >> >> I don't think there's a way around moving task_work out, just like it +hash_node. I was thinking to do apoll alloc+memcpy as for rw, but this one is ugly. >> was done on 5.9. The problem is that we could put the environment bits >> before doing task_work_add(), but we might need them if the subsequent >> queue ends up having to go async. So there's really no know when we can >> put them, outside of when the request finishes. Hence, we are kind of >> SOL here. > > Actually, if we do go async, then we can just grab the environment > again. We're in the same task at that point. So maybe it'd be better to > work on ensuring that the request is either in the valid work state, or > empty work if using task_work. > > Only potential complication with that is doing io_req_work_drop_env() > from the waitqueue handler, at least the ->needs_fs part won't like that > too much. Considering that work->list is removed before executing io_wq_work, it should work. And if done only for poll_add, which needs nothing and ends up with creds, there shouldn't be any problems. I'll try this out -- Pavel Begunkov
