Re: Kernel NBD client waits on wrong cookie, aborts connection

[Ming Lei <ming.lei@redhat.com>]

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@redhat.com> 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:


diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2cafcf11ee8b..e3eb31c3ee75 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -682,3 +682,16 @@ u32 blk_mq_unique_tag(struct request *rq)
 		(rq->tag & BLK_MQ_UNIQUE_TAG_MASK);
 }
 EXPORT_SYMBOL(blk_mq_unique_tag);
+
+/*
+ * Same with blk_mq_unique_tag, but one persistent tag is included in
+ * the request lifetime.
+ */
+u32 blk_mq_unique_static_tag(struct request *rq)
+{
+	u32 tag = rq->q->elevator ? rq->internal_tag : rq->tag;
+
+	return (rq->mq_hctx->queue_num << BLK_MQ_UNIQUE_TAG_BITS) |
+		(tag & BLK_MQ_UNIQUE_TAG_MASK);
+}
+EXPORT_SYMBOL(blk_mq_unique_static_tag);
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b852050d8a96..cc522a2cb9fb 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -201,7 +201,7 @@ static void nbd_requeue_cmd(struct nbd_cmd *cmd)
 static u64 nbd_cmd_handle(struct nbd_cmd *cmd)
 {
 	struct request *req = blk_mq_rq_from_pdu(cmd);
-	u32 tag = blk_mq_unique_tag(req);
+	u32 tag = blk_mq_unique_static_tag(req);
 	u64 cookie = cmd->cmd_cookie;
 
 	return (cookie << NBD_COOKIE_BITS) | tag;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 4fecf46ef681..d6266759d62d 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -793,6 +793,7 @@ enum {
 };
 
 u32 blk_mq_unique_tag(struct request *rq);
+u32 blk_mq_unique_static_tag(struct request *rq);
 
 static inline u16 blk_mq_unique_tag_to_hwq(u32 unique_tag)
 {

-- 
Ming