Re: [PATCH]: ufs: truncate should allocate block for last byte

[Evgeniy Dushistov <dushistov@mail.ru>]

On Wed, Jun 28, 2006 at 04:50:29AM -0700, Andrew Morton wrote:
> On Wed, 28 Jun 2006 13:38:51 +0400
> > +	if (unlikely(!page->mapping || !page_has_buffers(page))) {
> > +		unlock_page(page);
> > +		page_cache_release(page);
> > +		goto try_again;/*we really need these buffers*/
> > +	}
> > +out:
> > +	return page;
> > +}
> 
> I think there's a (preexisting) problem here.  When one thread is executing
> ufs_get_locked_page() while a second thread is running truncate().
> 
> If truncate got to the page first, truncate_complete_page() will mark the
> page !uptodate and will later unlock it.  Now this function gets the page
> lock and emits a printk (bad) and assumes -EIO (worse).
> 
> That scenario might not be possible because of i_mutex coverage, dunno.
> 
I suppose this is possible because of 
a)page may be mapped to hole
b)sys_msync doesn't use i_mutex
c)in case of block allocation we can call ufs_get_locked_page

> But if it _is_ possible, it can be simply fixed by doing
> 
But you added such check "!page->mapping" into ufs_get_locked_page,
is it not enough?

> 	lock_page(page);
> +	if (page->mapping == NULL) {
> +		/* truncate() got there first */
> +		page_cache_release(page);
> +		goto try_again;
> +	}
> 
> That's if it is appropriate to re-instantiate the page at a place which is
> now outside i_size...

-- 
/Evgeniy