Re: [PATCH-v2 1/2] fs: make sure the timestamps for lazytime inodes eventually get written

[Theodore Ts'o <tytso@mit.edu>]

On Mon, Mar 16, 2015 at 03:34:12PM -0600, Andreas Dilger wrote:
> I wonder if something more lightweight could be added to avoid this
> problem?  For example, we only care about this case if it has been
> going on for more than the lazytime interval (about a day), so the
> inode could store a 16-bit i_dirtied_time_when that is approximately
> (jiffies >> bits_in_a_half_a_day) and only check time_after() that.
> The __u16 could fit into some existing hole (e.g. after i_bytes on my
> kernel) and avoid expanding the size of the inode at all.
> 
> The remaining high bits of i_dirtied_time_when would be irrelevant, since
> a __u16 of half-days is about 80 years, so it would be enough to compare:
> 
> 
>     time_after(i_dirtied_time_when, (__u16)(jiffies >> bits_in_half_a_day))

That won't work correctly; we'd have to do something like this

#define u16_after(a,b) (typecheck(__u16, a) && typecheck(__u16, b) && \
		 ((__s16)((b) - (a)) < 0))


> Minor issue, is there a good reason why dirtied_time_when doesn't have an
> "i_" prefix?

It's because dirtied_when also doesn't have an i_ prefix, but arguably
it should.

						- Ted