linux-kernel - Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM

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Date:   Tue, 10 Nov 2020 12:21:11 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Stefan Agner <stefan@...er.ch>, Minchan Kim <minchan@...nel.org>,
        ngupta@...are.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        sjenning@...ux.vnet.ibm.com, gregkh <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Linux-MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/zsmalloc: include sparsemem.h for MAX_PHYSMEM_BITS

On Tue, Nov 10, 2020 at 10:58 AM Mike Rapoport <rppt@...nel.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.

> 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