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

[Peter Staubach]

Miklos Szeredi wrote:
> > > > > > > +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?
> >     
>
> Take this example:
>
>     fd = open()
>     addr = mmap(.., fd)
>     write(fd, ...)
>     close(fd)
>     sleep(100)
>     msync(addr,...)
>     munmap(addr)
>
> The file times will be updated in write(), but with your patch, the
> bit in the mapping will also be set.
>
> Then in msync() the file times will be updated again, which is wrong,
> since the memory was _not_ modified through the mapping.

This is correct.  I have updated my proposed patch to include the clearing
of AS_MCTIME in the routine which updates the mtime field.  I haven't
reposted it yet until I complete testing of the new resulting system.  I
anticipate doing this later today.

   Thanx..

      ps
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