Re: [PATCH] ceph: stop forwarding the request when exceeding 256 times

["Luís Henriques" <lhenriques@suse.de>]

xiubli@redhat.com writes:

> From: Xiubo Li <xiubli@redhat.com>
>
> The type of 'num_fwd' in ceph 'MClientRequestForward' is 'int32_t',
> while in 'ceph_mds_request_head' the type is '__u8'. So in case
> the request bounces between MDSes exceeding 256 times, the client
> will get stuck.
>
> In this case it's ususally a bug in MDS and continue bouncing the
> request makes no sense.

Ouch.  Nice catch.  This patch looks OK to me, just 2 minor comments
bellow.

> Signed-off-by: Xiubo Li <xiubli@redhat.com>
> ---
>  fs/ceph/mds_client.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index a89ee866ebbb..0bb6e7bc499c 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -3293,6 +3293,7 @@ static void handle_forward(struct ceph_mds_client *mdsc,
>  	int err = -EINVAL;
>  	void *p = msg->front.iov_base;
>  	void *end = p + msg->front.iov_len;
> +	bool aborted = false;
>  
>  	ceph_decode_need(&p, end, 2*sizeof(u32), bad);
>  	next_mds = ceph_decode_32(&p);
> @@ -3309,8 +3310,28 @@ static void handle_forward(struct ceph_mds_client *mdsc,
>  		dout("forward tid %llu aborted, unregistering\n", tid);
>  		__unregister_request(mdsc, req);
>  	} else if (fwd_seq <= req->r_num_fwd) {
> -		dout("forward tid %llu to mds%d - old seq %d <= %d\n",
> -		     tid, next_mds, req->r_num_fwd, fwd_seq);
> +		/*
> +		 * The type of 'num_fwd' in ceph 'MClientRequestForward'
> +		 * is 'int32_t', while in 'ceph_mds_request_head' the
> +		 * type is '__u8'. So in case the request bounces between
> +		 * MDSes exceeding 256 times, the client will get stuck.
> +		 *
> +		 * In this case it's ususally a bug in MDS and continue
> +		 * bouncing the request makes no sense.
> +		 */
> +		if (req->r_num_fwd == 256) {
> +			mutex_lock(&req->r_fill_mutex);
> +			req->r_err = -EIO;

Not sure -EIO is the most appropriate.  Maybe -E2BIG... although not quite
it either.

> +			set_bit(CEPH_MDS_R_ABORTED, &req->r_req_flags);
> +			mutex_unlock(&req->r_fill_mutex);
> +			aborted = true;
> +			dout("forward tid %llu to mds%d - seq overflowed %d <= %d\n",
> +			     tid, next_mds, req->r_num_fwd, fwd_seq);
> +			goto out;

This 'goto' statement can be dropped, but one before (when the
lookup_get_request() fails) needs to be adjusted, otherwise
ceph_mdsc_put_request() may be called with a NULL pointer.

Cheers,
-- 
Luís

> +		} else {
> +			dout("forward tid %llu to mds%d - old seq %d <= %d\n",
> +			     tid, next_mds, req->r_num_fwd, fwd_seq);
> +		}
>  	} else {
>  		/* resend. forward race not possible; mds would drop */
>  		dout("forward tid %llu to mds%d (we resend)\n", tid, next_mds);
> @@ -3322,9 +3343,13 @@ static void handle_forward(struct ceph_mds_client *mdsc,
>  		put_request_session(req);
>  		__do_request(mdsc, req);
>  	}
> -	ceph_mdsc_put_request(req);
>  out:
>  	mutex_unlock(&mdsc->mutex);
> +
> +	/* kick calling process */
> +	if (aborted)
> +		complete_request(mdsc, req);
> +	ceph_mdsc_put_request(req);
>  	return;
>  
>  bad:
> -- 
>
> 2.27.0
>