[PATCH] arm64: Use correct method to calculate nomap region boundaries

[Huacai Chen]

Hi, Will,

On Tue, Dec 14, 2021 at 3:07 AM Will Deacon <will at kernel.org> wrote:
>
> On Mon, Dec 06, 2021 at 12:46:27PM +0800, Huacai Chen wrote:
> > On Thu, Dec 2, 2021 at 6:08 PM Will Deacon <will at kernel.org> wrote:
> > > On Fri, Oct 22, 2021 at 03:06:46PM +0800, Huacai Chen wrote:
> > > > Nomap regions are treated as "reserved". When region boundaries are not
> > > > page aligned, we usually increase the "reserved" regions rather than
> > > > decrease them. So, we should use memblock_region_reserved_base_pfn()/
> > > > memblock_region_reserved_end_pfn() instead of memblock_region_memory_
> > > > base_pfn()/memblock_region_memory_base_pfn() to calculate boundaries.
> > > >
> > > > Signed-off-by: Huacai Chen <chenhuacai at loongson.cn>
> > > > ---
> > > >  arch/arm64/kernel/setup.c | 6 ++++--
> > > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> > > > index be5f85b0a24d..1e86d4c5ef8c 100644
> > > > --- a/arch/arm64/kernel/setup.c
> > > > +++ b/arch/arm64/kernel/setup.c
> > > > @@ -232,12 +232,14 @@ static void __init request_standard_resources(void)
> > > >               if (memblock_is_nomap(region)) {
> > > >                       res->name  = "reserved";
> > > >                       res->flags = IORESOURCE_MEM;
> > > > +                     res->start = __pfn_to_phys(memblock_region_reserved_base_pfn(region));
> > > > +                     res->end = __pfn_to_phys(memblock_region_reserved_end_pfn(region)) - 1;
> > > >               } else {
> > > >                       res->name  = "System RAM";
> > > >                       res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
> > > > +                     res->start = __pfn_to_phys(memblock_region_memory_base_pfn(region));
> > > > +                     res->end = __pfn_to_phys(memblock_region_memory_end_pfn(region)) - 1;
> > > >               }
> > > > -             res->start = __pfn_to_phys(memblock_region_memory_base_pfn(region));
> > > > -             res->end = __pfn_to_phys(memblock_region_memory_end_pfn(region)) - 1;
> > >
> > > Under which circumstances would the nomap regions not be page-aligned? That
> > > sounds like something we should prevent, rather than work around here.
> >
> > In this call path:
> > acpi_table_upgrade --> arch_reserve_mem_area --> memblock_mark_nomap
> > The nomap region is not page-aligned.
>
> Fair enough, then I think this patch is probably doing the right thing.
> However, Ard and Mark had some concerns about the new behaviour wrt
> adjacent nomap regions where the page containing the overlap would be
> reported twice. Is this a problem?
Seems no problem,  in most cases, memblock_mark_nomap() -->
memblock_merge_regions() --> memblock_merge_regions(), adjacent
intesected regions will be merged. Then the only problem is two or
more small regions not intersected but in a single page, I think this
is hardly happen..

But, even if this actually happens, I think it can be handle by the
later request_resource() and small regions will not be reported more
than once.

Huacai
>
> Will