Re: [PATCH] arm64: configurable sparsemem section size

[Anshuman Khandual]

On 04/24/2019 02:08 AM, Pavel Tatashin wrote:
> sparsemem section size determines the maximum size and alignment that
> is allowed to offline/online memory block. The bigger the size the less
> the clutter in /sys/devices/system/memory/*. On the other hand, however,
> there is less flexability in what granules of memory can be added and
> removed.

Is there any scenario where less than a 1GB needs to be added on arm64 ?

> 
> Recently, it was enabled in Linux to hotadd persistent memory that
> can be either real NV device, or reserved from regular System RAM
> and has identity of devdax.

devdax (even ZONE_DEVICE) support has not been enabled on arm64 yet.

> 
> The problem is that because ARM64's section size is 1G, and devdax must
> have 2M label section, the first 1G is always missed when device is
> attached, because it is not 1G aligned.

devdax has to be 2M aligned ? Does Linux enforce that right now ?

> 
> Allow, better flexibility by making section size configurable.

Unless 2M is being enforced from Linux not sure why this is necessary at
the moment.

> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig                 | 10 ++++++++++
>  arch/arm64/include/asm/sparsemem.h |  2 +-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b5d8cf57e220..a0c5b9d13a7f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -801,6 +801,16 @@ config ARM64_PA_BITS
>  	default 48 if ARM64_PA_BITS_48
>  	default 52 if ARM64_PA_BITS_52
>  
> +config ARM64_SECTION_SIZE_BITS
> +	int "sparsemem section size shift"
> +	range 27 30

27 and 28 do not even compile for ARM64_64_PAGES because of MAX_ORDER and
SECTION_SIZE mismatch.