Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

["Yan, Zheng" <ukernel@gmail.com>]

On Thu, Jun 1, 2017 at 6:22 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Thu, Jun 1, 2017 at 11:56 AM, Yan, Zheng <ukernel@gmail.com> wrote:
>> On Sat, Apr 8, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>
>>> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
>>> index 517838b..77204da 100644
>>> --- a/drivers/block/rbd.c
>>> +++ b/drivers/block/rbd.c
>>> @@ -1922,7 +1922,7 @@ static void rbd_osd_req_format_write(struct rbd_obj_request *obj_request)
>>>  {
>>>         struct ceph_osd_request *osd_req = obj_request->osd_req;
>>>
>>> -       osd_req->r_mtime = CURRENT_TIME;
>>> +       ktime_get_real_ts(&osd_req->r_mtime);
>>>         osd_req->r_data_offset = obj_request->offset;
>>>  }
>>>
>>> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
>>> index c681762..1d3fa90 100644
>>> --- a/fs/ceph/mds_client.c
>>> +++ b/fs/ceph/mds_client.c
>>> @@ -1666,6 +1666,7 @@ struct ceph_mds_request *
>>>  ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
>>>  {
>>>         struct ceph_mds_request *req = kzalloc(sizeof(*req), GFP_NOFS);
>>> +       struct timespec ts;
>>>
>>>         if (!req)
>>>                 return ERR_PTR(-ENOMEM);
>>> @@ -1684,7 +1685,8 @@ ceph_mdsc_create_request(struct ceph_mds_client *mdsc, int op, int mode)
>>>         init_completion(&req->r_safe_completion);
>>>         INIT_LIST_HEAD(&req->r_unsafe_item);
>>>
>>> -       req->r_stamp = current_fs_time(mdsc->fsc->sb);
>>> +       ktime_get_real_ts(&ts);
>>> +       req->r_stamp = timespec_trunc(ts, mdsc->fsc->sb->s_time_gran);
>>
>> This change causes our kernel_untar_tar test case to fail (inode's
>> ctime goes back). The reason is that there is time drift between the
>> time stamps got by  ktime_get_real_ts() and current_time(). We need to
>> revert this change until current_time() uses ktime_get_real_ts()
>> internally.
>
> Hmm, the change was not supposed to have a user-visible effect, so
> something has gone wrong, but I don't immediately see how it
> relates to what you observe.
>
> ktime_get_real_ts() and current_time() use the same time base, there
> is no drift, but there is a difference in resolution, as the latter uses
> the time stamp of the last jiffies update, which may be up to one jiffy
> (10ms) behind the exact time we put in the request stamps here.
>
> Do you still see problems if you use current_kernel_time() instead of
> ktime_get_real_ts()?

The problem disappears after using current_kernel_time().

https://github.com/ceph/ceph-client/commit/2e0f648da23167034a3cf1500bc90ec60aef2417


Regards
Yan, Zheng
>
>         Arnd