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

[Peter Staubach]

Miklos Szeredi wrote:
> > > > > Inspired by Peter Staubach's patch and the resulting comments.
> > > > >
> > > > >   
> > > > >       
> > > > >           
> > > > An updated version of the original patch was submitted to LKML
> > > > yesterday...  :-)
> > > >     
> > > >         
> > > Strange coincidence :)
> > >
> > >   
> > >       
> > > > >  		file = vma->vm_file;
> > > > >  		start = vma->vm_end;
> > > > > +		mapping_update_time(file);
> > > > >  		if ((flags & MS_SYNC) && file &&
> > > > >  				(vma->vm_flags & VM_SHARED)) {
> > > > >  			get_file(file);
> > > > >   
> > > > >       
> > > > >           
> > > > It seems to me that this might lead to file times being updated for
> > > > non-MAP_SHARED mappings.
> > > >     
> > > >         
> > > In theory no, because the COW-ed pages become anonymous and are not
> > > part of the original mapping any more.
> > >
> > >   
> > >       
> > I must profess to having a incomplete understanding of all of this
> > support, but then why would it be necessary to test VM_SHARED at
> > this point in msync()?
> >     
>
> That's basically just an optimization.  If it wasn't there, then data
> from a another (shared) mapping could be written back, which is not
> wrong, but not required either.
>
>   
> > I ran into problems early on with file times being updated incorrectly
> > so I am a little sensitive this aspect.
> >
> >     
> > > > > +int set_page_dirty_mapping(struct page *page);
> > > > >   
> > > > >       
> > > > >           
> > > > This aspect of the design seems intrusive to me.  I didn't see a strong
> > > > reason to introduce new versions of many of the routines just to handle
> > > > these semantics.  What motivated this part of your design?  Why the new
> > > > _mapping versions of routines?
> > > >     
> > > >         
> > > Because there's no way to know inside the set_page_dirty() functions
> > > if the dirtying comes from a memory mapping or from a modification
> > > through a normal write().  And they have different semantics, for
> > > write() the modification times are updated immediately.
> > >       
> > Perhaps I didn't understand what page_mapped() does, but it does seem to
> > have the right semantics as far as I could see.
> >     
>
> The problems will start, when you have a file that is both mapped and
> modified with write().  Then the dirying from the write() will set the
> flag, and that will have undesirable consequences.

I don't think that I quite follow the logic.  The dirtying from write()
will set the flag, but then the mtime will get updated and the flag will
be cleared by the hook in file_update_time().  Right?

   Thanx...

      ps
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