Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from_oldmem() for user pointer

[Matthew Wilcox]

On Mon, Dec 06, 2021 at 03:54:22PM +0100, Christoph Hellwig wrote:
> On Mon, Dec 06, 2021 at 02:17:24PM +0000, Matthew Wilcox wrote:
> > On Mon, Dec 06, 2021 at 03:04:51PM +0100, Christoph Hellwig wrote:
> > > This looks like a huge mess.  What speak against using an iov_iter
> > > here?
> > 
> > I coincidentally made a start on this last night.  Happy to stop.
> 
> Don't stop!
> 
> > What do you think to adding a generic copy_pfn_to_iter()?  Not sure
> > which APIs to use to implement it ... some architectures have weird
> > requirements about which APIs can be used for what kinds of PFNs.
> 
> Hmm.  I though kmap_local_pfn(_prot) is all we need?

In the !HIGHMEM case, that calls pfn_to_page(), and I think the
point of this path is that we don't have a struct page for this pfn.