Re: [PATCH 1/3] libceph: block I/O when PAUSE or FULL osd map flags are set

[Li Wang <liwang@ubuntukylin.com>]

I just had a quick look, did not think it thoroughly.
(1) If possible, there is a race condition, that a former write get 
blocked by FULL, a latter write is lucky to be sent to osd after FULL -> 
NOFULL,
then the former write is resent, to cause the old data overwrite the new 
data.
(2) If it keeps FULL, how long gotta the write request waiting? The 
upper fs writepage/writepages() kernel thread or sync process gotta hang 
there.


On 2013/12/4 7:12, Josh Durgin wrote:
> The PAUSEWR and PAUSERD flags are meant to stop the cluster from
> processing writes and reads, respectively. The FULL flag is set when
> the cluster determines that it is out of space, and will no longer
> process writes.  PAUSEWR and PAUSERD are purely client-side settings
> already implemented in userspace clients. The osd does nothing special
> with these flags.
>
> When the FULL flag is set, however, the osd responds to all writes
> with -ENOSPC. For cephfs, this makes sense, but for rbd the block
> layer translates this into EIO.  If a cluster goes from full to
> non-full quickly, a filesystem on top of rbd will not behave well,
> since some writes succeed while others get EIO.
>
> Fix this by blocking any writes when the FULL flag is set in the osd
> client. This is the same strategy used by userspace, so apply it by
> default.  A follow-on patch makes this configurable.
>
> __map_request() is called to re-target osd requests in case the
> available osds changed.  Add a paused field to a ceph_osd_request, and
> set it whenever an appropriate osd map flag is set.  Avoid queueing
> paused requests in __map_request(), but force them to be resent if
> they become unpaused.
>
> Also subscribe to the next osd map from the monitor if any of these
> flags are set, so paused requests can be unblocked as soon as
> possible.
>
> Fixes: http://tracker.ceph.com/issues/6079
>
> Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
> ---
>   include/linux/ceph/osd_client.h |    1 +
>   net/ceph/osd_client.c           |   29 +++++++++++++++++++++++++++--
>   2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/ceph/osd_client.h b/include/linux/ceph/osd_client.h
> index 8f47625..4fb6a89 100644
> --- a/include/linux/ceph/osd_client.h
> +++ b/include/linux/ceph/osd_client.h
> @@ -138,6 +138,7 @@ struct ceph_osd_request {
>   	__le64           *r_request_pool;
>   	void             *r_request_pgid;
>   	__le32           *r_request_attempts;
> +	bool              r_paused;
>   	struct ceph_eversion *r_request_reassert_version;
>
>   	int               r_result;
> diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
> index 2b4b32a..21476be 100644
> --- a/net/ceph/osd_client.c
> +++ b/net/ceph/osd_client.c
> @@ -1232,6 +1232,22 @@ void ceph_osdc_set_request_linger(struct ceph_osd_client *osdc,
>   EXPORT_SYMBOL(ceph_osdc_set_request_linger);
>
>   /*
> + * Returns whether a request should be blocked from being sent
> + * based on the current osdmap and osd_client settings.
> + *
> + * Caller should hold map_sem for read.
> + */
> +static bool __req_should_be_paused(struct ceph_osd_client *osdc,
> +				   struct ceph_osd_request *req)
> +{
> +	bool pauserd = ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_PAUSERD);
> +	bool pausewr = ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_PAUSEWR) ||
> +		ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_FULL);
> +	return (req->r_flags & CEPH_OSD_FLAG_READ && pauserd) ||
> +		(req->r_flags & CEPH_OSD_FLAG_WRITE && pausewr);
> +}
> +
> +/*
>    * Pick an osd (the first 'up' osd in the pg), allocate the osd struct
>    * (as needed), and set the request r_osd appropriately.  If there is
>    * no up osd, set r_osd to NULL.  Move the request to the appropriate list
> @@ -1248,6 +1264,7 @@ static int __map_request(struct ceph_osd_client *osdc,
>   	int acting[CEPH_PG_MAX_SIZE];
>   	int o = -1, num = 0;
>   	int err;
> +	bool was_paused;
>
>   	dout("map_request %p tid %lld\n", req, req->r_tid);
>   	err = ceph_calc_ceph_pg(&pgid, req->r_oid, osdc->osdmap,
> @@ -1264,12 +1281,18 @@ static int __map_request(struct ceph_osd_client *osdc,
>   		num = err;
>   	}
>
> +	was_paused = req->r_paused;
> +	req->r_paused = __req_should_be_paused(osdc, req);
> +	if (was_paused && !req->r_paused)
> +		force_resend = 1;
> +
>   	if ((!force_resend &&
>   	     req->r_osd && req->r_osd->o_osd == o &&
>   	     req->r_sent >= req->r_osd->o_incarnation &&
>   	     req->r_num_pg_osds == num &&
>   	     memcmp(req->r_pg_osds, acting, sizeof(acting[0])*num) == 0) ||
> -	    (req->r_osd == NULL && o == -1))
> +	    (req->r_osd == NULL && o == -1) ||
> +	    req->r_paused)
>   		return 0;  /* no change */
>
>   	dout("map_request tid %llu pgid %lld.%x osd%d (was osd%d)\n",
> @@ -1804,7 +1827,9 @@ done:
>   	 * we find out when we are no longer full and stop returning
>   	 * ENOSPC.
>   	 */
> -	if (ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_FULL))
> +	if (ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_FULL) ||
> +		ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_PAUSERD) ||
> +		ceph_osdmap_flag(osdc->osdmap, CEPH_OSDMAP_PAUSEWR))
>   		ceph_monc_request_next_osdmap(&osdc->client->monc);
>
>   	mutex_lock(&osdc->request_mutex);
>