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

On Tue 03-10-17 11:29:16, Pasha Tatashin wrote:

On 10/03/2017 09:18 AM, Michal Hocko wrote:

quoted

On Wed 20-09-17 16:17:10, Pavel Tatashin wrote:

quoted

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).

Then just initialize struct pages for that mapping rigth there where a
special API is used.

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)...

I have no idea wht empty_bad_page is but a quick grep shows that this is
never used. I might be wrong here but if somebody is reserving a memory
in a special way then we should handle the initialization right there.
E.g. create an API for special memblock reservations.
-- 
Michal Hocko
SUSE Labs