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

[Arnd Bergmann]

On Tue, Jul 14, 2020 at 1:06 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 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?

I was under the impression that the kdump kernel only accesses a small
amount of memory itself and would allocate its mem_map to fit that but
not pages of that were used by the kernel it is dumping.

       Arnd