Re: [PATCH] Fix incorrect kunmap_atomic in pktcdvd

[Andrew Morton <akpm@osdl.org>]

Peter Osterlund <petero2@telia.com> wrote:
>
> Andrew Morton <akpm@osdl.org> writes:
> 
> > Peter Osterlund <petero2@telia.com> wrote:
> > >
> > >  The pktcdvd driver uses kunmap_atomic() incorrectly. The function is
> > >  supposed to take an address as the first parameter, but the pktcdvd
> > >  driver passed a page pointer. Thanks to Douglas Gilbert and Jens Axboe
> > >  for discovering this.
> > 
> > You're about the 7,000th person to make that mistake.  We really should
> > catch it via typechecking but the code's really lame and nobody ever got
> > around to rotorooting it.
> 
> Why was the interface made different from kmap()/kunmap() in the first
> place? Wouldn't it have made more sense to let kunmap_atomic() take a
> page pointer as the first parameter?

No, kmap-atomic() maps a single page into the CPU's address space by making
a pte point at the page.  To unmap that page we need to get at the pte, not
at the page.  If kmap_atomic() were to take a pageframe address we'd need
to search the whole fixmap space for the corresponding page - a reverse
lookup.