Re: [Bugme-new] [Bug 2019] New: Bug from the mm subsystem involving X (fwd)

[Andrew Morton <akpm@osdl.org>]

Linus Torvalds <torvalds@osdl.org> wrote:
>
> 
> 
> On Wed, 4 Feb 2004, Martin J. Bligh wrote:
> > 
> > Oh hell ... I remember what's wrong with this whole bit. pfn_valid is
> > used inconsistently in different places, IIRC. Linus / Andrew ... what
> > do you actually want it to mean? Some things seem to use it to say
> > "the memory here is valid accessible RAM", some things "there is a 
> > valid struct page for this pfn". I was aiming for the latter, but a
> > few other arches seemed to disagree.
> > 
> > Could I get a ruling on this? ;-)
> 
> It _definitely_ means "there is a valid 'struct page' for this pfn". 
> 
> To test for "there is RAM" here, you need to first check that the pfn is
> valid, and then you can check what the page type is (usually that would be
> PageReserved(), but it could be a highmem check or something like that
> too).

pfn_valid() could become quite expensive indeed, and it lies on super-duper
hotpaths.

An alternative which is less conceptually clean but should work in this
case is to mark all vma's which were created by /dev/mem mappings as VM_IO,
and test that in remap_page_range().

The marking of mmap_mem() vma's as VM_IO has been in -mm for four months. 
But I didn't changelog it at the time and I've forgotten why I wrote it
(really).  It's something to do with get_user_pages() against a mapping of
/dev/mem :(

ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.2-rc3/2.6.2-rc3-mm1/broken-out/get_user_pages-handle-VM_IO.patch