Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 18 Oct 2023 at 10:41, Jeff Layton <jlayton@kernel.org> wrote:
>
> One way to prevent this is to ensure that when we stamp a file with a
> fine-grained timestamp, that we use that value to establish a floor for
> any later timestamp update.

I'm very leery of this.

I don't like how it's using a global time - and a global fine-grained
offset - when different filesystems will very naturally have different
granularities. I also don't like how it's no using that global lock.

Yes, yes, since the use of this all is then gated by the 'is_mgtime()'
thing, any filesystem with big granularities will presumably never set
FS_MGTIME in the first time, and that hides the worst pointless cases.
But it still feels iffy to me.

Also, the whole current_ctime() logic seems wrong. Later (in 4/9), you do this:

 static struct timespec64 current_ctime(struct inode *inode)
 {
        if (is_mgtime(inode))
                return current_mgtime(inode);

and current_mgtime() does

        if (nsec & I_CTIME_QUERIED) {
                ktime_get_real_ts64(&now);
                return timestamp_truncate(now, inode);
        }

so once the code has set I_CTIME_QUERIED, it will now use the
expensive fine-grained time - even when it makes no sense.

As far as I can tell, there is *never* a reason to get the
fine-grained time if the old inode ctime is already sufficiently far
away.

IOW, my gut feel is that all this logic should always not only be
guarded by FS_MGTIME (like you do seem to do), *and* by "has anybody
even queried this time" - it should *also* always be guarded by "if I
get the coarse-grained time, is that sufficient?"

So I think the current_ctime() logic should be something along the lines of

    static struct timespec64 current_ctime(struct inode *inode)
    {
        struct timespec64 ts64 = current_time(inode);
        unsigned long old_ctime_sec = inode->i_ctime_sec;
        unsigned int old_ctime_nsec = inode->i_ctime_nsec;

        if (ts64.tv_sec != old_ctime_sec)
                return ts64;

        /*
         * No need to check is_mgtime(inode) - the I_CTIME_QUERIED
         * flag is only set for mgtime filesystems
         */
        if (!(old_ctime_nsec & I_CTIME_QUERIED))
                return ts64;
        old_ctime_nsec &= ~I_CTIME_QUERIED;
        if (ts64.tv_nsec > old_ctime_nsec + inode->i_sb->s_time_gran)
                return ts64;

        /* Ok, only *now* do we do a finegrained value */
        ktime_get_real_ts64(&ts64);
        return timestamp_truncate(ts64);
    }

or whatever. Make it *very* clear that the finegrained timestamp is
the absolute last option, after we have checked that the regular one
isn't possible.

I dunno.

            Linus