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

[Michal Hocko]

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@xxxxxxxxxxxx> 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