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

[David Hildenbrand <david@redhat.com>]

On 25.07.2018 15:51, Michal Hocko wrote:
> On Tue 24-07-18 16:13:09, David Hildenbrand wrote:
> [...]
>> So I see right now:
>>
>> - Pg_reserved + e.g. new page type (or some other unique identifier in
>>   combination with Pg_reserved)
>>  -> Avoid reads of pages we know are offline
>> - extend is_ram_page()
>>  -> Fake zero memory for pages we know are offline
>>
>> Or even both (avoid reading and don't crash the kernel if it is being done).
> 
> I really fail to see how that can work without kernel being aware of
> PageOffline. What will/should happen if you run an old kdump tool on a
> kernel with this partially offline memory?
> 

New kernel with old dump tool:

a) we have not fixed up is_ram_page()

-> crash, as we access memory we shouldn't

b) we have fixed up is_ram_page()

-> We have a callback to check for applicable memory in the hypervisor
whether the parts are accessible / online or not accessible / offline.
(e.g. via a device driver that controls a certain memory region)

-> Don't read, but fake a page full of 0


So instead of the kernel being aware of it, it asks via is_ram_page()
the hypervisor.


I don't think a) is a problem. AFAICS, we have to update makedumpfile
for every new kernel. We can perform changes and update makedumpfile
to be compatible with new dump tools.

E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump
tools and required

commit 4bf4f2b0a855ccf4c7ffe13290778e92b2f5bbc9
Author: Pratyush Anand <panand@redhat.com>
Date:   Thu Aug 17 12:47:13 2017 +0900

    [PATCH v2] Fix SECTION_MAP_MASK for kernel >= v.13
    
    * Required for kernel 4.13
    
    commit 2d070eab2e82 "mm: consider zone which is not fully populated to
    have holes" added a new flag SECTION_IS_ONLINE and therefore
    SECTION_MAP_MASK has been changed. We are not able to find correct
    mem_map in makedumpfile for kernel version v4.13-rc1 and onward because
    of the above kernel change.
    
    This patch fixes the MASK value keeping the code backward compatible
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>


Same would apply for the new combination of PageReserved + X, where we
tell dump tools to exclude this page.

-- 

Thanks,

David / dhildenb