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

[David Hildenbrand]

On 26.07.2018 10:27, Michal Hocko wrote:
> On Wed 25-07-18 16:20:41, David Hildenbrand wrote:
>> 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
> 
> this is not acceptable, right? You do not want to crash your crash
> kernel ;)

Well, the same can happen today with PageHWPoison. The "new" kernel will
happily access such pages and crash as far as I understand (it has has
no idea of the old struct pages). Of course, this is "less likely" than
what I describe.

> 
>> 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 am still confused why do we even care about hypervisor. What if
> somebody wants to have partial memory hotplug on native OS?

Good point I was ignoring so far (too much focusing on my use case I
assume). So for these, we would have to catch illegal accesses and

a) report them (-EINVAL / - EIO) as you said
b) fake a zero page

I assume catching illegal accesses should be possible. Might require
some work across all architectures.

Still, dump tools should in addition not even try to read if possible.

>  
>> 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.
> 
> Not really. You simply do not crash the kernel just because you are
> trying to dump the already crashed kernel.
> 
>> E.g. remember SECTION_IS_ONLINE you introduced ? It broke dump
>> tools and required
> 
> But has it crashed the kernel when reading the dump? If yes then the
> whole dumping is fragile as hell...

No, I think it simply didn't work. At least that's what I assume ;) I
was rather saying that dump tools may have to be fixed up to work with a
new kernel.


-- 

Thanks,

David / dhildenb