Re: [patch resend v4] update ctime and mtime for mmaped write

Andrew Morton
2007-03-27T08:51:50Z

On Tue, 27 Mar 2007 10:28:16 +0200 Miklos Szeredi <miklos@szeredi.hu> wrote:

> > > But Peter Staubach says a RH custumer has files written thorugh mmap,
> > > which are not being backed up.
> >
> > Yes, I expect the backup problem is the major real-world hurt arising from
> > this bug.
> >
> > But I expect we could adequately plug that problem at munmap()-time. Or,
> > better, do_wp_page(). As I said - half-assed.
> >
> > It's a question if whether the backup problem is the only thing which is hurting
> > in the real-world, or if people have other problems.
> >
> > (In fact, what's wrong with doing it in do_wp_page()?
>
> It's rather more expensive, than just toggling a bit.

It shouldn't be, especially for filesystems which have one-second timestamp
granularity.

Filesystems which have s_time_gran=1 might hurt a bit, but no more than
they will with write().

Actually, no - we'd only update the mctime once per page per writeback
period (30 seconds by default) so the load will be small. It'll be more
often if the user is doing a lot of pte-cleaning via msync() or fsync(),
but then the m/ctime writes will be the least of their problems.

I'd have thought there were more substantial problems with something that
crude?