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

[Ard Biesheuvel]

On Mon, 13 Jul 2020 at 16:06, 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.

memremap(MEMREMAP_WB) will reuse the existing linear address if the PA
is covered by the linear mapping.

> 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.