Re: [PATCH v1 0/2] mm/kdump: exclude reserved pages in dumps

[Vlastimil Babka <vbabka@suse.cz>]

On 07/24/2018 01:19 PM, Michal Hocko wrote:
>> When creating a crashdump, I definitely need the pages containing memmap
>> included in the dump, so I can inspect the struct pages. But this is a
>> bit recursive issue, so I'll try making it clearer:
>>
>> 1) there are kernel pages with data (e.g. slab) that I typically need in
>> the dump, and are not PageReserved
>> 2) there are struct pages for pages 1) in the memmap that physically
>> hold the pageflags for 1), and these are PageReserved
>> 3) there are struct pages for pages 2) somewhere else in the memmap,
>> physically hold the pageflags for 2). They are probably also
>> PageReserved themselves ? and self-referencing.
>>
>> Excluding PageReserved from dump means there won't be cases 2) and 3) in
>> the dump, which at least for case 2) is making such dump almost useless
>> in many cases.
> 
> Yes, we cannot simply exclude all PageReserved pages. I was merely
> suggesting to rule out new special PageReserved pages that are denoting 
> offline pages. The same could be applied to HWPoison pages

So how about marking them with some "page type" that we got after
Matthew's struct page reorg? I assume the pages we're talking about are
in a state that they don't need the mapcount/mapping field or whatever
unions with the page type... but I guess some care would be needed to
not have false positives when the union field is actually used but
happens to look like the new type.