Re: [PATCH] update ctime and mtime for mmaped write

[Peter Staubach <staubach@redhat.com>]

Miklos Szeredi wrote:
> > On Wed, 21 Feb 2007 18:51:52 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> >     
> > > This patch makes writing to shared memory mappings update st_ctime and
> > > st_mtime as defined by SUSv3:
> > >
> > >    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.
> > >
> > > A new address_space flag is introduced: AS_CMTIME.  This is set each
> > > time a page is dirtied through a userspace memory mapping.  This
> > > includes write accesses via get_user_pages().
> > >
> > > Note, the flag is set unconditionally, even if the page is already
> > > dirty.  This is important, because the page might have been dirtied
> > > earlier by a non-mmap write.
> > >
> > > This flag is checked in msync() and __fput(), and if set, the file
> > > times are updated and the flag is cleared
> > >
> > > The flag is also cleared, if the time update is triggered by a normal
> > > write.  This is not mandated by the standard, but seems to be a sane
> > > thing to do.
> > >       
> > Why is the flag checked in __fput()?
> >     
>
> It's because of this bit in the standard:
>
>     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.
>
> It could be done in munmap/mremap, but it seemed more difficult to
> track down all the places where the vma is removed.  But yes, that may
> be a nicer solution.

It seems to me that, with this support, a file, which is mmap'd,
modified, but never msync'd or munmap'd, will never get its mtime
updated.  Or did I miss that?

I also don't see how an mmap'd block device will get its mtime
updated either.

   Thanx...

      ps
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