Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the pgprot_t in arch_add_memory()

[Logan Gunthorpe <logang@deltatee.com>]

On 2019-12-09 2:00 p.m., Dan Williams wrote:
>>>> Can we fiddle that into "struct mhp_restrictions" instead?
>>>
>>> Yes, if that's what people want, it's pretty trivial to do. I chose not
>>> to do it that way because it doesn't get passed down to add_pages() and
>>> it's not really a "restriction". If I don't hear any objections, I will
>>> do that for v2.
>>
>> I do agree that restriction is not the best fit. But I consider prot
>> argument to complicate the API to all users even though it is not really
>> clear whether we are going to have many users really benefiting from it.
>> Look at the vmalloc API and try to find how many users of __vmalloc do
>> not use PAGE_KERNEL.
> 
> At least for this I can foresee at least one more user in the
> pipeline, encrypted memory support for persistent memory mappings that
> will store the key-id in the ptes.
> 
>>
>> So I can see two options. One of them is to add arch_add_memory_prot
>> that would allow to have give and extra prot argument or simply call
>> an arch independent API to change the protection after arch_add_memory.
>> The later sounds like much less code. The memory shouldn't be in use by
>> anybody at that stage yet AFAIU. Maybe there even is an API like that.
> 
> I'm ok with passing it the same way as altmap or a new
> arch_add_memory_prot() my only hangup with after the fact changes is
> the wasted effort it inflicts in the init path for potentially large
> address ranges.

Yes, I'll change the way it's passed in for v2 as that seems to be
generally agreed upon. I can also add a patch to make the name change.

And, yes, given our testing, the wasted effort is quite significant so
I'm against changing the prots after the fact.

Logan