Re: mlockall and mmap of IO devices don't mix

[Ingo Oeser <ioe-lists@rameria.de>]

On Saturday 04 October 2003 07:47, David S. Miller wrote:
> On Fri, 3 Oct 2003 17:27:27 -0700
>
> Andrew Morton <akpm@osdl.org> wrote:
> > Maybe it's best to not overload VM_RESERVED in this manner, but to always
> > mark /dev/mem as VM_IO.
>
> Just in cast is isn't clear, it should be defined that anything
> that sets VM_IO on an mmap() area should prefill the page tables
> as a side effect of such a mmap() request.
>
> Then things like mlockall() have simple semantics on VM_IO area, they
> end up being a nop.

It should be already, but the check in get_user_pages() looks wrong and
will only disallow VM_IO if you provide space for an page array.
It should be unconditional and we are done, because follow_pages will
never be called with such vmas.

Putting this check for "forbidden VMAs" into follow_pages is also a good
decision, if follow_pages() is called by strange callers not knowing the
limitations. kernel/futex.c is such an user, which don't check these in
the fast path (does it like wait on hardware triggered futexes there?).

It might be good to add VM_RESERVED check to get_user_pages(), too.
These pages are available anyway, so never need to be considered from
its users.

Regards

Ingo Oeser