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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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


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

Christophe




> 
> Fixes: 1e8fed873e74 ("powerpc: drop ranges for definition of ARCH_FORCE_MAX_ORDER")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>   arch/powerpc/Kconfig | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> I plan to merge this via the powerpc fixes branch.
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 539d1f03ff42..bff5820b7cda 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -906,11 +906,17 @@ config DATA_SHIFT
>   
>   config ARCH_FORCE_MAX_ORDER
>   	int "Order of maximal physically contiguous allocations"
> +	range 7 8 if PPC64 && PPC_64K_PAGES
>   	default "8" if PPC64 && PPC_64K_PAGES
> +	range 12 12 if PPC64 && !PPC_64K_PAGES
>   	default "12" if PPC64 && !PPC_64K_PAGES
> +	range 8 10 if PPC32 && PPC_16K_PAGES
>   	default "8" if PPC32 && PPC_16K_PAGES
> +	range 6 10 if PPC32 && PPC_64K_PAGES
>   	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
>   	default "10"
>   	help
>   	  The kernel page allocator limits the size of maximal physically