[PATCH 86/87] fs: switch timespec64 fields in inode to discrete integers

[Arnd Bergmann]

On Thu, Sep 28, 2023, at 13:40, Jeff Layton wrote:
> On Thu, 2023-09-28 at 10:19 -0700, Darrick J. Wong wrote:
>>
>> > I remember seeing those patches go by. I don't remember that change
>> > being NaK'ed, but I wasn't paying close attention at the time 
>> > 
>> > Looking at it objectively now, I think it's worth it to recover 8 bytes
>> > per inode and open a 4 byte hole that Amir can use to grow the
>> > i_fsnotify_mask. We might even able to shave off another 12 bytes
>> > eventually if we can move to a single 64-bit word per timestamp. 
>> 
>> I don't think you can, since btrfs timestamps utilize s64 seconds
>> counting in both directions from the Unix epoch.  They also support ns
>> resolution:
>> 
>> 	struct btrfs_timespec {
>> 		__le64 sec;
>> 		__le32 nsec;
>> 	} __attribute__ ((__packed__));
>> 
>
> Correct. We'd lose some fidelity in currently stored timestamps, but as
> Linus and Ted pointed out, anything below ~100ns granularity is
> effectively just noise, as that's the floor overhead for calling into
> the kernel. It's hard to argue that any application needs that sort of
> timestamp resolution, at least with contemporary hardware. 

There are probably applications that have come up with creative
ways to use the timestamp fields of file systems that 94 bits
of data, with both the MSB of the seconds and the LSB of the
nanoseconds carrying information that they expect to be preserved.

Dropping any information in the nanoseconds other than the top two
bits would trivially change the 'ls -t' output when two files have
the same timestamp in one kernel but slightly different timestamps
in another one. For large values of 'tv_sec', there are fewer
obvious things that break, but if current kernels are able to
retrieve arbitrary times that were stored with utimensat(), then we
should probably make sure future kernels can see the same.

        Arnd