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

Linus Torvalds [off-list ref] wrote:

On Wed, 4 Feb 2004, Martin J. Bligh wrote:

quoted

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

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