Re: [PATCH 2 of 4] hotplug-memory: adding non-section-aligned memoryis bad

[Jeremy Fitzhardinge]

KAMEZAWA Hiroyuki wrote:
> On Thu, 27 Mar 2008 19:13:20 -0700
> Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>
>   
> > > > Because, firmware may occupy some area in the section.
> > > > Firmware must exclude those area to notify kernel. So, E820, EFI,
> > > > or _CRS of ACPI may return not aligned address and size.
> > > > register_memory_resource() and walk_memory_resource() are to skip
> > > > them silently. This is intended.
> > > >
> > > >     
> > > >         
> > > Ah, ok. sorry.
> > >
> > > Jeremy, I think you can check whether you have 'struct page' or not by
> > > pfn_valid(). 
> > >
> > > If pfn_valid() == false, you should call add_memory() and create
> > > a section/mem_map. If pfn_valid() == true, you should just remove
> > > PG_reserved bit in mem_map by online_page().
> > >       
> > OK.  Would that ever be necessary if I explicitly align my start and size?
> >
> >     
> Maybe no. but be carefull not to register resource in overlapped manner.
>   

Yes.  That's why I added add_memory_resource(), so I could use 
allocate_resource() to find a non-overlapping range to put the new memory.

> (I wrote online_page() in above, but online_pages() is maybe better.
>  It does all what you want.)
>   

No, for my use-case the pages must be onlined one by one as they get 
some physical memory assigned to them.  At the time I do add_memory(), 
I'm just allocating page structures, but there's no memory backing that 
range.

That's why I need to disable the sysfs onlining interface, because it 
bulk onlines the pages before there's anything behind them.

> Start/Size are automatically alined to section in __add_pages().
>
> See below.
> ==
> 110 int __add_pages(struct zone *zone, unsigned long phys_start_pfn,
>  111                  unsigned long nr_pages)
>  112 {
>  113         unsigned long i;
>  114         int err = 0;
>  115         int start_sec, end_sec;
>  116         /* during initialize mem_map, align hot-added range to section */
>  117         start_sec = pfn_to_section_nr(phys_start_pfn);
>  118         end_sec = pfn_to_section_nr(phys_start_pfn + nr_pages - 1);
> ==
>
> And online_pages(), which onlines pages in [pfn, pfn + size), will see
> registerred resources within [pfn, pfn + size).
> ==
> 184 int online_pages(unsigned long pfn, unsigned long nr_pages)
> <snip>
> 227         walk_memory_resource(pfn, nr_pages, &onlined_pages,
> 228                 online_pages_range);
> ==
>
> One of my concern is how-to-handle sysfs status in this case.
>
> Another concerns is, currently, I think no one tried to online a section twice
> to online reserved pages in a section. so, you may see bug.
> For example, mem_notify() in online_pages() will be called several times against
> a section.
>   

I'd really rather prevent online_pages from happening at all, since it 
can only cause havoc.

Thanks,
   J
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