On Tue, Oct 15, 2024 at 8:11 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > On Tue, Oct 15, 2024 at 08:01:43PM +0800, Ming Lei wrote: > > On Tue, Oct 15, 2024 at 6:22 PM Kevin Wolf <kwolf@xxxxxxxxxx> wrote: > > > > > > Hi all, > > > > > > the other day I was running some benchmarks to compare different QEMU > > > block exports, and one of the scenarios I was interested in was > > > exporting NBD from qemu-storage-daemon over a unix socket and attaching > > > it as a block device using the kernel NBD client. I would then run a VM > > > on top of it and fio inside of it. > > > > > > Unfortunately, I couldn't get any numbers because the connection always > > > aborted with messages like "Double reply on req ..." or "Unexpected > > > reply ..." in the host kernel log. > > > > > > Yesterday I found some time to have a closer look why this is happening, > > > and I think I have a rough understanding of what's going on now. Look at > > > these trace events: > > > > > > qemu-img-51025 [005] ..... 19503.285423: nbd_header_sent: nbd transport event: request 000000002df03708, handle 0x0000150c0000005a > > > [...] > > > qemu-img-51025 [008] ..... 19503.285500: nbd_payload_sent: nbd transport event: request 000000002df03708, handle 0x0000150c0000005d > > > [...] > > > kworker/u49:1-47350 [004] ..... 19503.285514: nbd_header_received: nbd transport event: request 00000000b79e7443, handle 0x0000150c0000005a > > > > > > This is the same request, but the handle has changed between > > > nbd_header_sent and nbd_payload_sent! I think this means that we hit one > > > of the cases where the request is requeued, and then the next time it > > > is executed with a different blk-mq tag, which is something the nbd > > > driver doesn't seem to expect. > > > > > > Of course, since the cookie is transmitted in the header, the server > > > replies with the original handle that contains the tag from the first > > > call, while the kernel is only waiting for a handle with the new tag and > > > is confused by the server response. > > > > > > I'm not sure yet which of the following options should be considered the > > > real problem here, so I'm only describing the situation without trying > > > to provide a patch: > > > > > > 1. Is it that blk-mq should always re-run the request with the same tag? > > > I don't expect so, though in practice I was surprised to see that it > > > happens quite often after nbd requeues a request that it actually > > > does end up with the same cookie again. > > > > No. > > > > request->tag will change, but we may take ->internal_tag(sched) or > > ->tag(none), which won't change. > > > > I guess was_interrupted() in nbd_send_cmd() is triggered, then the payload > > is sent with a different tag. > > > > I will try to cook one patch soon. > > Please try the following patch: Oops, please ignore the patch, it can't work since nbd_handle_reply() doesn't know static tag. Thanks,
