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

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Message-Id: <20070327005150.9177ae02.akpm@linux-foundation.org>
Date:	Tue, 27 Mar 2007 00:51:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch resend v4] update ctime and mtime for mmaped write

On Tue, 27 Mar 2007 10:28:16 +0200 Miklos Szeredi <miklos@...redi.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?
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