quoted

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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.

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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.

Hi Michal,

The reservations happen before struct pages are allocated and mapped. 
So, it is not always possible to do it at call sites.

Previously, I have solved this problem like this:

https://patchwork.kernel.org/patch/9886163

But, I was not too happy with that approach, so I replaced it with the 
current approach as it is more generic, and solves similar issues if 
they happen in other places. Also, the comment in page-flags got me 
scared that there are probably other places perhaps on other 
architectures that can have the similar issue.

In addition, I did not like my solution, I was simply shrinking the low 
reservation from:
[0 - reserve_low) to [min_pfn - reserve_low), but if min_pfn > 
reserve_low can we skip low reservation entirely? I was not sure.

The current approach notifies us if there are such pages, and we can 
fix/remove them in the future without crashing kernel in the meantime.

Pasha