Re: [PATCH 5/7] mm/mm_init.c: Extend init unavailable range doc info

[Serge Semin]

Hi Mike

On Thu, Nov 23, 2023 at 12:18:54PM +0200, Mike Rapoport wrote:
> Hi Serge,
> 
> On Wed, Nov 22, 2023 at 09:24:03PM +0300, Serge Semin wrote:
> > Besides of the already described reasons the pages backended memory holes
> > might be persistent due to having memory mapped IO spaces behind those
> > ranges in the framework of flatmem kernel config. Add such note to the
> > init_unavailable_range() method kdoc in order to point out to one more
> > reason of having the function executed for such regions.
> > 
> > Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>
> > 
> > ---
> > 
> > Please let me know if the IO-space pages must be initialized somehow
> > differently rather relying on free_area_init() executing the
> > init_unavailable_range() method.
> 

> Maybe I'm missing something, but why do you need struct pages in the
> IO space?

In my case at the very least that's due to having a SRAM device
available in the middle of the MMIO-space. The region is getting
mapped using the ioremap_wc() method (Uncached Write-Combine CA),
which eventually is converted to calling get_vm_area() and
ioremap_page_range() (see ioremap_prot() function on MIPS), which in
its turn use the page structs for mapping. Another similar case is
using ioremap_wc() in the PCIe outbound ATU space mapping of
the graphic/video cards framebuffers.

In general having the pages array defined for the IO-memory is
required for mapping the IO-space other than just uncached (my sram
case for example) or, for instance, with special access attribute for
the user-space (if I am not missing something in a way VM works in
that case).

-Serge(y)

> 
> > ---
> >  mm/mm_init.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > index 077bfe393b5e..3fa33e2d32ba 100644
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -796,6 +796,7 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
> >   * - physical memory bank size is not necessarily the exact multiple of the
> >   *   arbitrary section size
> >   * - early reserved memory may not be listed in memblock.memory
> > + * - memory mapped IO space
> >   * - memory layouts defined with memmap= kernel parameter may not align
> >   *   nicely with memmap sections
> >   *
> > -- 
> > 2.42.1
> > 
> 
> -- 
> Sincerely yours,
> Mike.
>