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

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Date:   Sun, 4 Jun 2017 15:54:27 +0800
From:   "Yan, Zheng" <ukernel@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
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>,
        David Miller <davem@...emloft.net>,
        David Sterba <dsterba@...e.com>,
        Evgeniy Dushistov <dushistov@...l.ru>,
        Eric Paris <eparis@...hat.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Josef Bacik <jbacik@...com>,
        Jeff Layton <jlayton@...chiereds.net>,
        James Simmons <jsimmons@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Drokin, Oleg" <oleg.drokin@...el.com>,
        Paul Moore <paul@...l-moore.com>,
        Steven Rostedt <rostedt@...dmis.org>, yuchao0@...wei.com,
        ceph-devel <ceph-devel@...r.kernel.org>,
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Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

On Fri, Jun 2, 2017 at 10:18 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Fri, Jun 2, 2017 at 2:18 PM, Yan, Zheng <ukernel@...il.com> wrote:
>> On Fri, Jun 2, 2017 at 7:33 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>> On Fri, Jun 2, 2017 at 1:18 PM, Yan, Zheng <ukernel@...il.com> wrote:
>>> 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.
>
> Ok, I see it now, thanks for the clarification. Most other file systems do this
> the other way round and update all fields in the in-memory inode structure
> first and then write that to persistent storage, so I was getting confused about
> the order of events here.
>
> If I understand it all right, we have three different behaviors in ceph now,
> though the differences are very minor and probably don't ever matter:
>
> - in setattr(), we update ctime in the in-memory inode first and then send
>   the same time to the mds, and expect to set it again when the reply comes.
>
> - in ceph_write_iter write() and mmap/page_mkwrite(), we call
>   file_update_time() to set i_mtime and i_ctime to the same
>   timestamp first once a write is observed by the fs and then take
>   two other timestamps that we send to the mds, and update the
>   in-memory inode a second time when the reply comes. ctime
>   is never older than mtime here, as far as I can tell, but it may
>   be newer when the timer interrupt happens between taking the
>   two stamps.

We don't use request to send i_mtime/i_ctime to mds in this case.
Instead, we use cap flush message. i_mtime/i_ctime are directly
encoded in cap flush message. When mds receives the cap flush message,
it writes i_mtime/i_ctime to persistent storage and sends a cap flush
ack message to client. (when client receives the cap flush ack
message, it does not update i_mtime/i_ctime). There is no issue as you
described.

>
> - in all other calls, we only update the inode (and/or parent inode)
>   after the reply arrives.

There are two cases. 1. Client updates in-memory inode's ctime, it
sends the new ctime to mds through cap flush message. 2. client set
mds request's r_stamp and send the request to mds. MDS updates
relavent inodes' ctime and sends reply to client. Client updates
in-memory inodes' ctime according to the reply.

Regards
Yan, Zheng

>
>        Arnd