Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

[Pasha Tatashin]

On 10/03/2017 09:18 AM, Michal Hocko wrote:
> On Wed 20-09-17 16:17:10, Pavel Tatashin wrote:
> > Some memory is reserved but unavailable: not present in memblock.memory
> > (because not backed by physical pages), but present in memblock.reserved.
> > Such memory has backing struct pages, but they are not initialized by going
> > through __init_single_page().
>
> Could you be more specific where is such a memory reserved?

I know of one example: trim_low_memory_range() unconditionally reserves 
from pfn 0, but e820__memblock_setup() might provide the exiting memory 
from pfn 1 (i.e. KVM).

But, there could be more based on this comment from linux/page-flags.h:

 19  * PG_reserved is set for special pages, which can never be swapped 
out. Some
 20  * of them might not even exist (eg empty_bad_page)...

Pasha