Rather than printing a warning at mount time (which may be confusing to users for a problem they may never see), it makes sense to only print such a warning in the vanishingly small case that someone actually tries to modify the inode timestamp but it doesn't fit, rather than on the theoretical case that may never happen.

Arnd and I were discussing and we came to a similar conclusion that we would not warn at mount. Arnd suggested maybe we include a pr_warn_ratelimited() when we do write to these special inodes.

In general, there is an agreement to leave the fs granularity to a higher resolution for such super blocks. And hence, the timestamps for these special cases will never be clamped in memory.(Assuming we do not want to make more changes and try to do something like __ext4_expand_extra_isize() for in memory inode updates) We could easily try to clamp these timestamps when they get written out to the disk by modifying the EXT4_INODE_SET_XTIME to include such a clamp. And, at this point we could also warn.

If this is acceptable, I could post an update.

-Deepa