Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM_BITS

[Mike Rapoport <rppt@kernel.org>]

On Tue, Nov 10, 2020 at 12:21:11PM +0100, Arnd Bergmann wrote:
> On Tue, Nov 10, 2020 at 10:58 AM Mike Rapoport <rppt@kernel.org> wrote:
> > > >
> > > > asm/sparsemem.h is not available on some architectures.
> > > > It's better to use linux/mmzone.h instead.
> 
> Ah, I missed that, too.
> 
> > > Hm, linux/mmzone.h only includes asm/sparsemem.h when CONFIG_SPARSEMEM
> > > is enabled. However, on ARM at least I can have configurations without
> > > CONFIG_SPARSEMEM and physical address extension on (e.g.
> > > multi_v7_defconfig + CONFIG_LPAE + CONFIG_ZSMALLOC).
> > >
> > > While sparsemem seems to be a good idea with LPAE it really seems not
> > > required (see also https://lore.kernel.org/patchwork/patch/567589/).
> > >
> > > There seem to be also other architectures which define MAX_PHYSMEM_BITS
> > > only when SPARSEMEM is enabled, e.g.
> > > arch/riscv/include/asm/sparsemem.h...
> > >
> > > Not sure how to get out of this.. Maybe make ZSMALLOC dependent on
> > > SPARSEMEM? It feels a bit silly restricting ZSMALLOC selection only due
> > > to a compile time define...
> >
> > I think we can define MAX_POSSIBLE_PHYSMEM_BITS in one of
> > arch/arm/inclide/asm/pgtable-{2,3}level-*.h headers to values supported
> > by !LPAE and LPAE.
> 
> Good idea. I wonder what other architectures need the same though.
> Here are some I found:
> 
> $ git grep -l PHYS_ADDR_T_64BIT arch | grep Kconfig
> arch/arc/Kconfig
> arch/arm/mm/Kconfig
> arch/mips/Kconfig
> arch/powerpc/platforms/Kconfig.cputype
> arch/x86/Kconfig
> 
> arch/arc has a CONFIG_ARC_HAS_PAE40 option
> arch/riscv has 34-bit addressing in rv32 mode
> arch/mips has up to 40 bits with mips32r3 XPA, but I don't know what
>     supports that
> 
> arch/powerpc has this:
> config PHYS_64BIT
>         bool 'Large physical address support' if E500 || PPC_86xx
>         depends on (44x || E500 || PPC_86xx) && !PPC_83xx && !PPC_82xx
> 
> Apparently all three (4xx, e500v2, mpc86xx/e600) do 36-bit physical
> addressing, but each one has a different page table format.
> 
> Microblaze has physical address extensions, but neither those nor
> 64-bit mode have so far made it into the kernel.
> 
> To be on the safe side, we could provoke a compile-time error
> when CONFIG_PHYS_ADDR_T_64BIT is set on a 32-bit
> architecture, but MAX_POSSIBLE_PHYSMEM_BITS is not set.

Maybe compile time warning and a runtime error in zs_init() if 32 bit
machine has memory above 4G?

> > That's what x86 does:
> >
> > $ git grep -w MAX_POSSIBLE_PHYSMEM_BITS arch/
> > arch/x86/include/asm/pgtable-3level_types.h:#define MAX_POSSIBLE_PHYSMEM_BITS   36
> 
> Doesn't x86 also support a 40-bit addressing mode? I suppose
> those machines that actually used it are long gone.
> 
> > arch/x86/include/asm/pgtable_64_types.h:#define MAX_POSSIBLE_PHYSMEM_BITS       52
> >
> > It seems that actual numbers would be 36 for !LPAE and 40 for LPAE, but
> > I'm not sure about that.
> 
> Close enough, yes.
> 
> The 36-bit addressing is on !LPAE is only used for early static mappings,
> so I think we can pretend it's always 32-bit. I checked the ARMv8 reference,
> and it says that ARMv8-Aarch32 actually supports 40 bit physical addressing
> both with non-LPAE superpages (short descriptor format) and LPAE (long
> descriptor format), but Linux only does 36-bit addressing on superpages
> as specified for ARMv6/ARMv7 short descriptors.
> 
>        Arnd

-- 
Sincerely yours,
Mike.