Re: [parisc-linux] Re: Example filesystem fail to init on parisc

On Thu, 2006-03-16 at 09:13 -0700, Grant Grundler wrote:
> On Mon, Mar 13, 2006 at 12:26:34PM +0100, Miklos Szeredi wrote:
> > OK.  I'm out of ideas, CC-ing parisc-linux, maybe they can help shed
> > some light on this.
> 
> Miklos,
> I took a quick look last week but didn't see anything obvious.
> I was suspicious of the memcopy() calls to copy stuff to
> user space. But willy me told those are fine if using the
> result of kmap() (and then kunmap'ing when done).

Sorry, meant to reply earlier.

> > The reason appears to be that when the userspace filesystem reads from
> > the FUSE device, the kernel doesn't copy any data to the userspace
> > read buffer, though the correct size is returned by read().
> > 
> > FUSE uses a combination of get_user_pages(), kmap_atomic() and
> > memcpy().  After kunmap_atomic(), flush_dcache_page() is called to
> > avoid virtual aliasing.  The data in the read buffer is totally
> > untouched.

This is wrong.  A VIPT cache requires a mapping to flush on.  If you
kunmap, you've lost the mapping.  What you should do is

kmap()
operate on data
flush_kernel_dcache_page()
kunmap()

James

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