Re: [PATCH 1/3] lib: Add a generic copy_oldmem_page()

[Christoph Hellwig]

On Mon, Jul 13, 2020 at 03:39:17PM +0200, Arnd Bergmann wrote:
> On Mon, Jul 13, 2020 at 3:07 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > On Fri, Jul 10, 2020 at 08:55:42PM -0700, Palmer Dabbelt wrote:
> > > +ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
> > > +                      size_t csize, unsigned long offset,
> > > +                      int userbuf)
> > > +{
> > > +     void *vaddr;
> > > +
> > > +     if (!csize)
> > > +             return 0;
> > > +
> > > +     vaddr = memremap(__pfn_to_phys(pfn), PAGE_SIZE, MEMREMAP_WB);
> > > +     if (!vaddr)
> > > +             return -ENOMEM;
> >
> > Doing a memremap for every page is very inefficient.  Also I don't see
> > why you'd want to even do that.  All memory is in the direct mapping
> > for RISC-V.  For other architecture that support highmem kmap_atomic_pfn
> > would do the job, which is what I'd use in a generic version.
> 
> I would expect the 'oldmem' data to not have a 'struct page', which would
> be a problem at least for the generic implementation of kmap_atomic_pfn()

Why do you expect it to not have a struct page?