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

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Message-ID: <CAK8P3a1etuRz6zwytXTHqK-T0g2YnbR4zcCwo98ehjjD1Dj+Ow@mail.gmail.com>
Date:   Fri, 2 Jun 2017 16:18:26 +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>,
        David Miller <davem@...emloft.net>,
        David Sterba <dsterba@...e.com>,
        Evgeniy Dushistov <dushistov@...l.ru>,
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        Paul Moore <paul@...l-moore.com>,
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Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

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.

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

       Arnd