Re: [V2] powerpc/Kconfig: Update config option based on page size.

[Balbir Singh <bsingharora@gmail.com>]

On 14/09/16 20:40, santhosh wrote:
> 
>> Michael Ellerman <mpe@ellerman.id.au> writes:
>>
>>> On Fri, 2016-19-02 at 05:38:47 UTC, Rashmica Gupta wrote:
>>>> Currently on PPC64 changing kernel pagesize from 4K to 64K leaves
>>>> FORCE_MAX_ZONEORDER set to 13 - which produces a compile error.
>>>>
>>> ...
>>>> So, update the range of FORCE_MAX_ZONEORDER from 9-64 to 8-9 for 64K pages
>>>> and from 13-64 to 9-13 for 4K pages.
>>>>
>>>> Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
>>>> Reviewed-by: Balbir Singh <bsingharora@gmail.com>
>>> Applied to powerpc next, thanks.
>>>
>>> https://git.kernel.org/powerpc/c/a7ee539584acf4a565b7439cea
>>>
>> HPAGE_PMD_ORDER is not something we should check w.r.t 4k linux page
>> size. We do have the below constraint w.r.t hugetlb pages
>>
>> static inline bool hstate_is_gigantic(struct hstate *h)
>> {
>>     return huge_page_order(h) >= MAX_ORDER;
>> }
>>
>> That require MAX_ORDER to be greater than 12.
>>

9 to 13 was done based on calculations you can find the commit



>> Did we test hugetlbfs 4k config with this patch ? Will it work if we
>> start marking hugepage as gigantic page ?
>>
>> -aneesh
>>
> Hello Rashmica,
> 
> With upstream linux kernel 4.8.0-rc1-00006-gbae9cc6 compiled with linux 4k page size we are not able set hugepages, Aneesh had a look at the problem and he mentioned this commit is causing the issue.
> 
> *Details:*
> We are using pkvm ubuntu 16.04 guest with upstream kernel [4.8.0-rc1-00006-gbae9cc6] compiled with  4k page size
> 
> o/p from guest:
> HugePages_Total:       0
> HugePages_Free:        0
> HugePages_Rsvd:        0
> HugePages_Surp:        0
> Hugepagesize:      16384 kB
> 
> Page sizes from device-tree: [dmesg]
> [    0.000000] base_shift=12: shift=12, sllp=0x0000, avpnm=0x00000000, tlbiel=1, penc=0
> [    0.000000] base_shift=12: shift=24, sllp=0x0000, avpnm=0x00000000, tlbiel=1, penc=56
> [    0.000000] base_shift=24: shift=24, sllp=0x0100, avpnm=0x00000001, tlbiel=0, penc=0
> 
> while trying to configure the hugepages inside the guest it throws the below error:
> 
> echo 100 > /proc/sys/vm/nr_hugepages
> -bash: echo: write error: Invalid argument
> 
> *Note*: we do not see the problem when the linux page is 64k


Just to reiterate you are seeing this problem using 4k page size and 16M as the hugepage size.
With FORCE_MAX_ZONEORDER set to 9 to 13 for 4k pages, you can do upto 32M if FORCE_MAX_ZONEORDER
is 13 and same for 64K with FORCE_MAX_ZONEORDER set to 9.

Basically the constraint is


FORCE_MAX_ZONEBITS <= 25 - PAGESHIFT

What is your value of FORCE_MAX_ZONEORDER in the .config?

Balbir Singh.