On Tue, Apr 05, 2022 at 08:20:24PM -0600, Jens Axboe wrote: > On 4/3/22 5:45 AM, Ming Lei wrote: > > -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is > > set in io_complete_rw_iopoll(), but the req never gets chance to be handled. > > io_iopoll_check doesn't handle this situation, and io hang can be caused. > > > > Current dm io polling may return -EAGAIN after bio submission is > > returned, also blk-throttle might trigger this situation too. > > I don't think this is necessarily safe. Handling REQ_F_ISSUE from within > the issue path is fine, as the request hasn't been submitted yet and > hence we know that passed in structs are still stable. Once you hit it > when polling for it, the io_uring_enter() call to submit requests has > potentially already returned, and now we're in a second call where we > are polling for requests. If we're doing eg an IORING_OP_READV, the > original iovec may no longer be valid and we cannot safely re-import > data associated with it. Yeah, this reissue is really not safe, thanks for the input. I guess the only way is to complete the cqe for this situation. > > Hence I don't think the patch is safe and we cannot reliably handle this > scenario. dm would need to retry internally for this. Another scenario might be bio throttle, which may delay submit_bio into kthrotld wq context, where request allocation can return -EAGAIN too. But I don't reproduce the problem in this scenario yet. Thanks, Ming
