linux-kernel - Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the pgprot_t in arch_add

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Message-ID: <a9d6cfe8-39fb-accf-acdc-7cce5578bf2f@redhat.com>
Date:   Tue, 10 Dec 2019 12:25:46 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-ia64@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux-sh <linux-sh@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the pgprot_t
 in arch_add_memory()

On 10.12.19 11:34, Michal Hocko wrote:
> On Tue 10-12-19 11:09:46, David Hildenbrand wrote:
>> On 10.12.19 11:04, Michal Hocko wrote:
>>> On Mon 09-12-19 12:43:40, Dan Williams wrote:
>>>> On Mon, Dec 9, 2019 at 12:24 PM Logan Gunthorpe <logang@...tatee.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 2019-12-09 12:23 p.m., David Hildenbrand wrote:
>>>>>> On 09.12.19 20:13, Logan Gunthorpe wrote:
>>> [...]
>>>>>>>  #ifdef CONFIG_MEMORY_HOTPLUG
>>>>>>> -int arch_add_memory(int nid, u64 start, u64 size,
>>>>>>> +int arch_add_memory(int nid, u64 start, u64 size, pgprot_t prot,
>>>>>>>                      struct mhp_restrictions *restrictions)
>>>>>>
>>>>>> 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.
>>>>
>>>> +1 to storing this information alongside the altmap in that structure.
>>>> However, I agree struct mhp_restrictions, with the MHP_MEMBLOCK_API
>>>> flag now gone, has lost all of its "restrictions". How about dropping
>>>> the 'flags' property and renaming the struct to 'struct
>>>> mhp_modifiers'?
>>>
>>> Hmm, this email somehow didn't end up in my inbox so I have missed it
>>> before replying.
>>>
>>> Well, mhp_modifiers makes some sense and it would reduce the API
>>> proliferation but how do you expect the prot part to be handled?
>>> I really do not want people to think about PAGE_KERNEL or which
>>> protection to use because my experience tells that this will get copied
>>> without much thinking or simply will break with some odd usecases.
>>> So how exactly this would be used?
>>
>> I was thinking about exactly the same "issue".
>>
>> 1. default initialization via a function
>>
>> memhp_modifier_default_init(&modified);
>>
>> 2. a flag that unlocks the prot field (default:0). Without the flag, it
>> is ignored. We can keep the current initialization then.
>>
>> Other ideas?
> 
> 3. a prot mask to apply on top of PAGE_KERNEL? Or would that be
> insufficient/clumsy?
> 

If it works for the given use case, I guess this would be simple and ok.

-- 
Thanks,

David / dhildenb