Re: [PATCH] powerpc/mm: Reinstate ARCH_FORCE_MAX_ORDER ranges

[Michael Ellerman <mpe@ellerman.id.au>]

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> Le 19/05/2023 à 13:38, Michael Ellerman a écrit :
>> Commit 1e8fed873e74 ("powerpc: drop ranges for definition of
>> ARCH_FORCE_MAX_ORDER") removed the limits on the possible values for
>> ARCH_FORCE_MAX_ORDER.
>>
>> However removing the ranges entirely causes some common work flows to
>> break. For example building a defconfig (which uses 64K pages), changing
>> the page size to 4K, and rebuilding used to work, because
>> ARCH_FORCE_MAX_ORDER would be clamped to 12 by the ranges.
>>
>> With the ranges removed it creates a kernel that builds but crashes at
>> boot:
>>    kernel BUG at mm/huge_memory.c:470!
>>    Oops: Exception in kernel mode, sig: 5 [#1]
>>    ...
>>    NIP hugepage_init+0x9c/0x278
>>    LR  do_one_initcall+0x80/0x320
>>    Call Trace:
>>      do_one_initcall+0x80/0x320
>>      kernel_init_freeable+0x304/0x3ac
>>      kernel_init+0x30/0x1a0
>>      ret_from_kernel_user_thread+0x14/0x1c
>>
>> The reasoning for removing the ranges was that some of the values were
>> too large. So take that into account and limit the maximums to 10 which
>> is the default max, except for the 4K case which uses 12.
>
> There is something wrong:
>
> ~# echo 1 > /sys/kernel/mm/hugepages/hugepages-8192kB/nr_hugepages
> sh: write error: Invalid argument
>
> $ grep -e MAX_ORDER -e K_PAGES .config
> CONFIG_PPC_4K_PAGES=y
> # CONFIG_PPC_16K_PAGES is not set
> CONFIG_ARCH_FORCE_MAX_ORDER=10

That's 32-bit I guess?

So you're hitting the default case, ie. range 10 10.

I see we have:

  arch/powerpc/configs/fsl-emb-nonhw.config:CONFIG_ARCH_FORCE_MAX_ORDER=12

And that config fragment is included by:
  mpc85xx_defconfig
  mpc85xx_smp_defconfig
  corenet32_smp_defconfig
  corenet64_smp_defconfig
  mpc86xx_defconfig
  mpc86xx_smp_defconfig

So for a lot of platforms the "default" was actually 12 in the past,
despite what it says in Kconfig.

> In the past MAX_ORDER used to be 12 but now it is force to 10.

This should fix it?

cheers


diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 0b1172cbeccb..b3fdb3d26836 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -917,7 +917,7 @@ config ARCH_FORCE_MAX_ORDER
 	default "6" if PPC32 && PPC_64K_PAGES
 	range 4 10 if PPC32 && PPC_256K_PAGES
 	default "4" if PPC32 && PPC_256K_PAGES
-	range 10 10
+	range 10 12
 	default "10"
 	help
 	  The kernel page allocator limits the size of maximal physically