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

Arnd Bergmann
2018-03-15T09:05:05Z

On Thu, Mar 15, 2018 at 7:11 AM, Andiry Xu <jix024@eng.ucsd.edu> wrote:
> On Wed, Mar 14, 2018 at 9:54 PM, Darrick J. Wong
> <darrick.wong@oracle.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.

Arnd