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

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-03-29 at 19:12 +0800, Xiubo Li wrote:
> On 3/29/22 5:53 PM, Luís Henriques wrote:
> > 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.
> > 
> Yeah, I also not very sure here.
> 
> Jeff ?
> 

Matching errors like this really comes down to a judgement call.  E2BIG
usually means that some buffer was sized too small, so you'll have users
trying to figure out what they passed in wrong if you return that here.

-EIO is the usual "default" when you don't know what else to use.
There's also -EREMOTEIO which may be closer here since this is
indicative of MDS problems. Given that, it may also be a good idea to
log a pr_warn or pr_notice message at the same time explaining what
happened.


> 
> > > +			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.
> 
> Yeah, will fix it.
> 
> Thanks.
> 
> -- Xiubo
> 
> 
> > Cheers,
> 

-- 
Jeff Layton <jlayton@kernel.org>