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Date:   Tue, 10 Dec 2019 11:34:52 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-ia64@vger.kernel.org,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Linux-sh <linux-sh@vger.kernel.org>,
        platform-driver-x86@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the
 pgprot_t in arch_add_memory()
Message-ID: <20191210103452.GF10404@dhcp22.suse.cz>
References: <20191209191346.5197-1-logang@deltatee.com>
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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@deltatee.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?
-- 
Michal Hocko
SUSE Labs