Re: Thoughts on simple scanner approach for free page hinting

[David Hildenbrand <david@redhat.com>]

On 08.04.19 22:10, Alexander Duyck wrote:
> On Mon, Apr 8, 2019 at 11:40 AM David Hildenbrand <david@redhat.com> wrote:
>>
>>>>>
>>>>> In addition we will need some way to identify which pages have been
>>>>> hinted on and which have not. The way I believe easiest to do this
>>>>> would be to overload the PageType value so that we could essentially
>>>>> have two values for "Buddy" pages. We would have our standard "Buddy"
>>>>> pages, and "Buddy" pages that also have the "Offline" value set in the
>>>>> PageType field. Tracking the Online vs Offline pages this way would
>>>>> actually allow us to do this with almost no overhead as the mapcount
>>>>> value is already being reset to clear the "Buddy" flag so adding a
>>>>> "Offline" flag to this clearing should come at no additional cost.
>>>>
>>>> Just nothing here that this will require modifications to kdump
>>>> (makedumpfile to be precise and the vmcore information exposed from the
>>>> kernel), as kdump only checks for the the actual mapcount value to
>>>> detect buddy and offline pages (to exclude them from dumps), they are
>>>> not treated as flags.
>>>>
>>>> For now, any mapcount values are really only separate values, meaning
>>>> not the separate bits are of interest, like flags would be. Reusing
>>>> other flags would make our life a lot easier. E.g. PG_young or so. But
>>>> clearing of these is then the problematic part.
>>>>
>>>> Of course we could use in the kernel two values, Buddy and BuddyOffline.
>>>> But then we have to check for two different values whenever we want to
>>>> identify a buddy page in the kernel.
>>>
>>> Actually this may not be working the way you think it is working.
>>
>> Trust me, I know how it works. That's why I was giving you the notice.
>>
>> Read the first paragraph again and ignore the others. I am only
>> concerned about makedumpfile that has to be changed.
>>
>> PAGE_OFFLINE_MAPCOUNT_VALUE
>> PAGE_BUDDY_MAPCOUNT_VALUE
>>
>> Once you find out how these values are used, you should understand what
>> has to be changed and where.
> 
> Ugh. Is there an official repo I am supposed to refer to for makedumpfile?
> 
> As far as the changes needed I don't think this would necessitate
> additional exports. We could probably just get away with having
> makedumpfile generate a new value by simply doing an "&" of the two
> values to determine what an offline buddy would be. If need be I can
> submit a patch for that. I find it kind of annoying that the kernel is
> handling identifying these bits one way, and makedumpfile is doing it
> another way. It should have been setup to handle this all the same
> way.

Here you go:

https://sourceforge.net/p/makedumpfile/code/ci/master/tree/

for now we only had one type at a time, so it wasn't an issue. E.g.
Buddy or Offline were never used in combination with other types. They
had distinct mapcount values.

-- 

Thanks,

David / dhildenb