linux-kernel - Re: [PATCH] arm64: limit MAX_PHYSMEM

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Message-ID: <Zoas9V3sLEOzULhs@arm.com>
Date: Thu, 4 Jul 2024 15:08:53 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: D Scott Phillips <scott@...amperecomputing.com>,
	linux-arm-kernel@...ts.infradead.org, Will Deacon <will@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-kernel@...r.kernel.org, patches@...erecomputing.com
Subject: Re: [PATCH] arm64: limit MAX_PHYSMEM_BITS based on vmemmap

On Thu, Jul 04, 2024 at 08:42:52AM +0530, Anshuman Khandual wrote:
> On 7/4/24 02:37, D Scott Phillips wrote:
> > diff --git a/arch/arm64/include/asm/sparsemem.h b/arch/arm64/include/asm/sparsemem.h
> > index 8a8acc220371c..8387301f2e206 100644
> > --- a/arch/arm64/include/asm/sparsemem.h
> > +++ b/arch/arm64/include/asm/sparsemem.h
> > @@ -5,7 +5,7 @@
> >  #ifndef __ASM_SPARSEMEM_H
> >  #define __ASM_SPARSEMEM_H
> >  
> > -#define MAX_PHYSMEM_BITS	CONFIG_ARM64_PA_BITS
> > +#define MAX_PHYSMEM_BITS	ilog2(VMEMMAP_RANGE)
> 
> Just wondering if there is another method, which avoids selecting physical
> memory ranges not backed with vmemmap. Also will reducing MAX_PHYSMEM_BITS
> below ARM64_PA_BITS have other side effects ? Do other platforms have this
> exact same co-relation between MAX_PHYSMEM_BITS and vmemmap range ?

That's indeed a pretty weird workaround. MAX_PHYSMEM_BITS, as the name
implies, is about the physical bits supported for memory while
VMEMMAP_RANGE tells us the virtual address range. There is a correlation
between them but they are different things conceptually.

The memory hotplug code uses arch_get_mappable_range(). This should be
called from the amdgpu code rather than changing MAX_PHYSMEM_BITS.

-- 
Catalin