Re: [PATCH v2 2/2] crash_core, vmcoreinfo: Append 'MAX_PHYSMEM_BITS' to vmcoreinfo

[Bhupesh Sharma]

Hi Kazu,

On 03/13/2019 01:17 AM, Kazuhito Hagio wrote:
> Hi Bhupesh,
>
> -----Original Message-----
> > Right now user-space tools like 'makedumpfile' and 'crash' need to rely
> > on a best-guess method of determining value of 'MAX_PHYSMEM_BITS'
> > supported by underlying kernel.
> >
> > This value is used in user-space code to calculate the bit-space
> > required to store a section for SPARESMEM (similar to the existing
> > calculation method used in the kernel implementation):
> >
> >    #define SECTIONS_SHIFT    (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
> >
> > Now, regressions have been reported in user-space utilities
> > like 'makedumpfile' and 'crash' on arm64, with the recently added
> > kernel support for 52-bit physical address space, as there is
> > no clear method of determining this value in user-space
> > (other than reading kernel CONFIG flags).
> >
> > As per suggestion from makedumpfile maintainer (Kazu), it makes more
> > sense to append 'MAX_PHYSMEM_BITS' to vmcoreinfo in the core code itself
> > rather than in arch-specific code, so that the user-space code for other
> > archs can also benefit from this addition to the vmcoreinfo and use it
> > as a standard way of determining 'SECTIONS_SHIFT' value in user-land.
> >
> > A reference 'makedumpfile' implementation which reads the
> > 'MAX_PHYSMEM_BITS' value from vmcoreinfo in a arch-independent fashion
> > is available here:
> >
> > [0]. https://github.com/bhupesh-sharma/makedumpfile/blob/remove-max-phys-mem-bit-v1/arch/ppc64.c#L471
> >
> > Cc: Boris Petkov <bp@xxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: James Morse <james.morse@xxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Dave Anderson <anderson@xxxxxxxxxx>
> > Cc: Kazuhito Hagio <k-hagio@xxxxxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: kexec@xxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> > ---
> >   kernel/crash_core.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> > index 093c9f917ed0..44b90368e183 100644
> > --- a/kernel/crash_core.c
> > +++ b/kernel/crash_core.c
> > @@ -467,6 +467,7 @@ static int __init crash_save_vmcoreinfo_init(void)
> >   #define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
> >   	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
> >   #endif
> > +	VMCOREINFO_NUMBER(MAX_PHYSMEM_BITS);
>
> Some architectures define MAX_PHYSMEM_BITS only with CONFIG_SPARSEMEM,
> so we need to move this to the #ifdef section that exports some
> mem_section things.
>
> Thanks!
> Kazu

Sorry for the late response, I wanted to make sure I check almost all  
archs to understand if a proposal would work for all.

As per my current understanding, we can protect the export of  
'MAX_PHYSMEM_BITS' via a #ifdef section against CONFIG_SPARSEMEM, and it  
should work for all archs. Here are some arguments to support the same,  
would request maintainers of various archs (in Cc) to correct me if I am  
missing something here:

1. SPARSEMEM is dependent upon on (!SELECT_MEMORY_MODEL &&  
ARCH_SPARSEMEM_ENABLE) || SPARSEMEM_MANUAL:

config SPARSEMEM
	def_bool y
	depends on (!SELECT_MEMORY_MODEL && ARCH_SPARSEMEM_ENABLE) ||  
SPARSEMEM_MANUAL

2. For a couple of archs, this option is already turned on by default in  
their respective defconfigs:

$ grep -nrw "CONFIG_SPARSEMEM_MANUAL" *
arch/ia64/configs/gensparse_defconfig:18:CONFIG_SPARSEMEM_MANUAL=y
arch/powerpc/configs/ppc64e_defconfig:30:CONFIG_SPARSEMEM_MANUAL=y

3. Note that other archs use ARCH_SPARSEMEM_DEFAULT to define if  
CONFIG_SPARSEMEM_MANUAL is set by default:

choice
	prompt "Memory model"
        ..
	default SPARSEMEM_MANUAL if ARCH_SPARSEMEM_DEFAULT

3a.

$ grep -nrw -A 2 "ARCH_SPARSEMEM_DEFAULT" *
arch/s390/Kconfig:621:config ARCH_SPARSEMEM_DEFAULT
arch/s390/Kconfig-622-	def_bool y
--
arch/x86/Kconfig:1623:config ARCH_SPARSEMEM_DEFAULT
arch/x86/Kconfig-1624-	def_bool y
arch/x86/Kconfig-1625-	depends on X86_64
--
arch/powerpc/Kconfig:614:config ARCH_SPARSEMEM_DEFAULT
arch/powerpc/Kconfig-615-	def_bool y
arch/powerpc/Kconfig-616-	depends on PPC_BOOK3S_64
--
arch/arm64/Kconfig:850:config ARCH_SPARSEMEM_DEFAULT
arch/arm64/Kconfig-851-	def_bool ARCH_SPARSEMEM_ENABLE
--
arch/sh/mm/Kconfig:138:config ARCH_SPARSEMEM_DEFAULT
arch/sh/mm/Kconfig-139-	def_bool y
--
arch/sparc/Kconfig:315:config ARCH_SPARSEMEM_DEFAULT
arch/sparc/Kconfig-316-	def_bool y if SPARC64
--
arch/arm/Kconfig:1591:config ARCH_SPARSEMEM_DEFAULT
arch/arm/Kconfig-1592-	def_bool ARCH_SPARSEMEM_ENABLE

Since most archs (except MIPS) set  
CONFIG_ARCH_SPARSEMEM_DEFAULT/CONFIG_ARCH_SPARSEMEM_ENABLE to y in the  
default configurations, so even though they don't protect  
'MAX_PHYSMEM_BITS' define in CONFIG_SPARSEMEM ifdef sections, we still  
would be ok protecting the 'MAX_PHYSMEM_BITS' vmcoreinfo export inside a  
CONFIG_SPARSEMEM ifdef section.

Thanks for your inputs, I will include this change in the v3.

Regards,
Bhupesh