On Thu, Mar 15, 2018 at 2:05 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > On Thu, Mar 15, 2018 at 7:11 AM, Andiry Xu <jix024@xxxxxxxxxxxx> wrote: >> On Wed, Mar 14, 2018 at 9:54 PM, Darrick J. Wong >> <darrick.wong@xxxxxxxxxx> 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
