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

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

On Fri, Jun 2, 2017 at 7:33 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Fri, Jun 2, 2017 at 1:18 PM, Yan, Zheng <ukernel@gmail.com> wrote:
>> On Fri, Jun 2, 2017 at 6:51 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>>> On Fri, Jun 2, 2017 at 12:10 PM, Yan, Zheng <ukernel@gmail.com> wrote:
>>>> On Fri, Jun 2, 2017 at 5:45 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>>>>> On Fri, Jun 2, 2017 at 4:09 AM, Yan, Zheng <ukernel@gmail.com> wrote:
>>>>>> On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>>>>>>> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@linaro.org> wrote:
>>>>>>>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@gmail.com> wrote:
>>>>>
>>>>> I believe the bug you see is the result of the two timestamps
>>>>> currently being almost guaranteed to be different in the latest
>>>>> kernels.
>>>>> Changing r_stamp to use current_kernel_time() will make it the
>>>>> same value most of the time (as it was before Deepa's patch),
>>>>> but when the timer interrupt happens between the timestamps,
>>>>> the two are still different, it's just much harder to hit.
>>>>>
>>>>> I think the proper solution should be to change __ceph_setattr()
>>>>> in a way that has req->r_stamp always synchronized with i_ctime.
>>>>> If we copy i_ctime to r_stamp, that will also take care of the
>>>>> future issues with the planned changes to current_time().
>>>>>
>>>> I already have a patch
>>>> https://github.com/ceph/ceph-client/commit/24f54cd18e195a002ee3d2ab50dbc952fd9f82af
>>>
>>> Looks good to me. In case anyone cares:
>>> Acked-by: Arnd Bergmann <arnd@arndb.de>
>>>
>>>>> The part I don't understand is what else r_stamp (i.e. the time
>>>>> stamp in ceph_msg_data with type==
>>>>> CEPH_MSG_CLIENT_REQUEST) is used for, other than setting
>>>>> ctime in CEPH_MDS_OP_SETATTR.
>>>>>
>>>>> Will this be used to update the stored i_ctime for other operations
>>>>> too? If so, we would need to synchronize it with the in-memory
>>>>> i_ctime for all operations that do this.
>>>>>
>>>>
>>>> yes,  mds uses it to update ctime of modified inodes. For example,
>>>> when handling mkdir, mds set ctime of both parent inode and new inode
>>>> to r_stamp.
>>>
>>> I see, so we may have a variation of that problem there as well: From
>>> my reading of the code, the child inode is not in memory yet, so
>>> that seems fine, but I could not find where the parent in-memory inode
>>> i_ctime is updated in ceph, but it is most likely not the same as
>>> req->r_stamp (assuming it gets updated at all).
>>
>> i_ctime is updated when handling request reply, by ceph_fill_file_time().
>> __ceph_setattr() can update the in-memory inode's ctime after request
>> reply is received. The difference between ktime_get_real_ts() and
>> current_time() can be larger than round-trip time of request. So it's
>> still possible that __ceph_setattr() make ctime go back.
>
> But the __ceph_setattr() problem should be fixed by your patch, right?
>
> What I meant is another related problem in ceph_mkdir() where the
> i_ctime field of the parent inode is different between the persistent
> representation in the mds and the in-memory representation.
>

I don't see any problem in mkdir case. Parent inode's i_ctime in mds is set to
r_stamp. When client receives request reply, it set its in-memory inode's ctime
to the same time stamp.

Regards
Yan, Zheng

>     Arnd
>
>>> Would it make sense require all callers of ceph_mdsc_do_request()
>>> to update r_stamp at the same time as i_ctime to keep them in sync?