linux-kernel - Re: [PATCH][RFC][BUG] updating the ctime and mtime time stamps in msync()

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Message-ID: <9a8748490801090641s41a06c1era3764091f135567d@mail.gmail.com>
Date:	Wed, 9 Jan 2008 15:41:11 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Anton Salikhmetov" <salikhmetov@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, joe@...lesco.com
Subject: Re: [PATCH][RFC][BUG] updating the ctime and mtime time stamps in msync()

On 09/01/2008, Anton Salikhmetov <salikhmetov@...il.com> wrote:
> Since no reaction in LKML was recieved for this message it seemed
> logical to suggest closing the bug #2645 as "WONTFIX":
>
> http://bugzilla.kernel.org/show_bug.cgi?id=2645#c15
>
> However, the reporter of the bug, Jacob Oestergaard, insisted the
> solution to be resubmitted once more:
>

Good idea. The bug is real and should be fixed IMHO.


...
> This bug causes backup systems to *miss* changed files.
>
> This bug does cause data loss in common real-world deployments (I gave an
> example with a database when posting the bug, but this affects the data from
> all mmap using applications with common backup systems).
>
Not just backup systems, but any application that relies on mtime
being correctly updated will be bitten by this.


> Silent exclusion from backups is very very nasty.
>
Agreed.

In fact if mtime is not reliable (which it is not) one could argue
that we might as well not update it at all, ever. But I think we can
all agree that just fixing it (as your patch does) is a lot better.

> Please comment on my solution or commit it if it's acceptable in its
> present form.
>
I've only looked briefly at your patch but it seems resonable. I'll
try to do some testing with it later.

Thank you for working on this long standing bug.

...
> > I would like to propose my solution for the bug #2645 from the kernel bug tracker:
> >
> > http://bugzilla.kernel.org/show_bug.cgi?id=2645
> >
> > The Open Group defines the behavior of the mmap() function as follows.
> >
> > The st_ctime and st_mtime fields of a file that is mapped with MAP_SHARED
> > and PROT_WRITE shall be marked for update at some point in the interval
> > between a write reference to the mapped region and the next call to msync()
> > with MS_ASYNC or MS_SYNC for that portion of the file by any process.
> > If there is no such call and if the underlying file is modified as a result
> > of a write reference, then these fields shall be marked for update at some
> > time after the write reference.
> >
> > The above citation was taken from the following link:
> >
> > http://www.opengroup.org/onlinepubs/009695399/functions/mmap.html
> >
...

I agree that our current behaviour is certainly not what the standard
(sensibly) requires.


-- 
Jesper Juhl <jesper.juhl@...il.com>
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