Re: [PATCH] mm: Improve comment before pagecache_isize_extended()

[Jan Kara <jack@suse.cz>]

On Tue 04-11-14 12:20:26, Jan Beulich wrote:
> >>> On 04.11.14 at 12:43, <"jack@suse.cz".non-mime.internet> wrote:
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -743,10 +743,13 @@ EXPORT_SYMBOL(truncate_setsize);
> >   * changed.
> >   *
> >   * The function must be called after i_size is updated so that page fault
> > - * coming after we unlock the page will already see the new i_size.
> > - * The function must be called while we still hold i_mutex - this not only
> > - * makes sure i_size is stable but also that userspace cannot observe new
> > - * i_size value before we are prepared to store mmap writes at new inode size.
> > + * coming after we unlock the page will already see the new i_size.  The caller
> > + * must make sure (generally by holding i_mutex but e.g. XFS uses its private
> > + * lock) i_size cannot change from the new value while we are called. It must
> > + * also make sure userspace cannot observe new i_size value before we are
> > + * prepared to store mmap writes upto new inode size (otherwise userspace could
> > + * think it stored data via mmap within i_size but they would get zeroed due to
> > + * writeback & reclaim because they have no backing blocks).
> >   */
> >  void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to)
> >  {
> 
> May I suggest that the comment preceding truncate_setsize() also be
> updated/removed?
  But that comment is actually still true AFAICT because VFS takes i_mutex
before calling into ->setattr(). So we hold i_mutex in truncate_setsize()
even for XFS.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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