linux-kernel - Re: [RFC v2 03/83] Add super.h.

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Date:   Thu, 15 Mar 2018 10:51:46 -0700
From:   Andiry Xu <jix024@....ucsd.edu>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Rudoff, Andy" <andy.rudoff@...el.com>, coughlan@...hat.com,
        Steven Swanson <swanson@...ucsd.edu>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
        swhiteho@...hat.com, miklos@...redi.hu,
        Jian Xu <andiry.xu@...il.com>, Andiry Xu <jix024@...ucsd.edu>
Subject: Re: [RFC v2 03/83] Add super.h.

On Thu, Mar 15, 2018 at 2:05 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Thu, Mar 15, 2018 at 7:11 AM, Andiry Xu <jix024@....ucsd.edu> wrote:
>> On Wed, Mar 14, 2018 at 9:54 PM, Darrick J. Wong
>> <darrick.wong@...cle.com> wrote:
>>> On Sat, Mar 10, 2018 at 10:17:44AM -0800, Andiry Xu wrote:
>
>>>> +     /* s_mtime and s_wtime should be together and their order should not be
>>>> +      * changed. we use an 8 byte write to update both of them atomically
>>>> +      */
>>>> +     __le32          s_mtime;                /* mount time */
>>>> +     __le32          s_wtime;                /* write time */
>>>
>>> Hmmm, 32-bit timestamps?  2038 isn't that far away...
>>>
>>
>> I will try fixing this in the next version.
>
> I would also recommend adding nanosecond-resolution timestamps.
> In theory, a signed 64-bit nanosecond field is sufficient for each timestamp
> (it's good for several hundred years), but the more common format uses
> 64-bit seconds and 32-bit nanoseconds in other file systems.
>
> Unfortunately it looks, you will have to come up with a more sophisticated
> update method above, even if you leave out the nanoseconds, you can't
> easily rely on a 16-byte atomic update across architectures to deal with
> the two 64-bit timestamps. For the superblock fields, you might be able
> to get away with using second resolution, and then encoding the
> timestamps as a signed 64-bit 'mkfs time' along with two unsigned
> 32-bit times added on top, which gives you a range of 136 years mount
> a file system after its creation.
>

I will take a look at other file systems.

Superblock mtime is not a big problem as it is updated rarely. 64-bit
seconds and 32-bit nanoseconds make the inode and log entry bigger,
and updating file->atime cannot be done with a single 64bit update.
That may be annoying and needs to use journaling.

Thanks,
Andiry

>       Arnd