On Wed, Nov 2, 2022 at 7:56 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote: > > Hi, > > 在 2022/11/03 9:39, Khazhismel Kumykov 写道: > > This fixes crashes in bfq_add_bfqq_busy due to waker_bfqq being NULL, > > but woken_list_node still being hashed. This would happen when > > bfq_init_rq() expects a brand new allocated queue to be returned from > > From what I see, bfqq->waker_bfqq is updated in bfq_init_rq() only if > 'new_queue' is false, but if 'new_queue' is false, the returned 'bfqq' > from bfq_get_bfqq_handle_split() will never be oom_bfqq, so I'm confused > here... There's two calls for bfq_get_bfqq_handle_split in this function - the second one is after the check you mentioned, and is the problematic one. > > > bfq_get_bfqq_handle_split() and unconditionally updates waker_bfqq > > without resetting woken_list_node. Since we can always return oom_bfqq > > when attempting to allocate, we cannot assume waker_bfqq starts as NULL. > > We must either reset woken_list_node, or avoid setting woken_list at all > > for oom_bfqq - opt to do the former. > > Once oom_bfqq is used, I think the io is treated as issued from root > group. Hence I don't think it's necessary to set woken_list or > waker_bfqq for oom_bfqq. Ack, I was wondering what's right here since, evidently, *someone* had already set oom_bfqq->waker_bfqq to *something* (although... maybe it was an earlier init_rq). But maybe it's better to do nothing if we *know* it's oom_bfqq. Is it a correct interpretation here that setting waker_bfqq won't accomplish anything anyways on oom_bfqq, so better off not? > > Thanks, > Kuai
