linux-kernel - Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real

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Message-ID: <CAK8P3a3m3nt6rfvAvDG6HFuUvy+qWbmk5K0UtLGqYhYHOfdF8w@mail.gmail.com>
Date:   Fri, 2 Jun 2017 13:33:32 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Yan, Zheng" <ukernel@...il.com>
Cc:     Deepa Dinamani <deepa.kernel@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        gregkh <gregkh@...uxfoundation.org>,
        "Dilger, Andreas" <andreas.dilger@...el.com>,
        "J. Bruce Fields" <bfields@...ldses.org>, Chris Mason <clm@...com>,
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        David Sterba <dsterba@...e.com>,
        Evgeniy Dushistov <dushistov@...l.ru>,
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Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

On Fri, Jun 2, 2017 at 1:18 PM, Yan, Zheng <ukernel@...il.com> wrote:
> On Fri, Jun 2, 2017 at 6:51 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Fri, Jun 2, 2017 at 12:10 PM, Yan, Zheng <ukernel@...il.com> wrote:
>>> On Fri, Jun 2, 2017 at 5:45 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>>> On Fri, Jun 2, 2017 at 4:09 AM, Yan, Zheng <ukernel@...il.com> wrote:
>>>>> On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@...il.com> wrote:
>>>>>> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@...aro.org> wrote:
>>>>>>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@...il.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@...db.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.

    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?