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

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Date:   Thu, 12 Nov 2020 11:49:57 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>, Stefan Agner <stefan@...er.ch>,
        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

Hi,

On Wed, Nov 11, 2020 at 08:52:00AM +0200, Mike Rapoport wrote:
> Hi,
> 
> On Tue, Nov 10, 2020 at 03:36:20PM -0800, Minchan Kim wrote:
> > On Tue, Nov 10, 2020 at 06:21:55PM +0200, Mike Rapoport wrote:
> > > 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@...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.
> > > 
> > > Maybe compile time warning and a runtime error in zs_init() if 32 bit
> > > machine has memory above 4G?
> > 
> > I guess max_pfn will represent maximum pfn configued in the system
> > and will not be changed in the runtime. If it's true, how about this?
> > (didn't test at all but just for RFC)
> 
> Largely, max_pfn is the maximal pfn at boot time but I don't think it
> can be used on systems with memory hotplug. Unless I'm missing
> something, with memory hotplug we must have compile-time maximum.
> 

Make sense. I am looking forward to seeing Arnd's patches at this
moment.

Thanks!

Thanks for the help!