Re: [PATCH RFC 2/8] mm: introduce PG_offline

[David Hildenbrand]

On 22.04.2018 16:02, Matthew Wilcox wrote:
> On Sun, Apr 22, 2018 at 10:17:31AM +0200, David Hildenbrand wrote:
>> On 22.04.2018 05:01, Matthew Wilcox wrote:
>>> On Sat, Apr 21, 2018 at 06:52:18PM +0200, Vlastimil Babka wrote:
>>>> Sounds like your newly introduced "page types" could be useful here? I
>>>> don't suppose those offline pages would be using mapcount which is
>>>> aliased there?
>>>
>>> Oh, that's a good point!  Yes, this is a perfect use for page_type.
>>> We have something like twenty bits available there.
>>>
>>> Now you've got me thinking that we can move PG_hwpoison and PG_reserved
>>> to be page_type flags too.  That'll take us from 23 to 21 bits (on 32-bit,
>>> with PG_UNCACHED)
>>
>> Some things to clarify here. I modified the current RFC to also allow
>> PG_offline on allocated (ballooned) pages (e.g. virtio-balloon).
>>
>> kdump based dump tools can then easily identify which pages are not to
>> be dumped (either because the content is invalid or not accessible).
>>
>> I previously stated that ballooned pages would be marked as PG_reserved,
>> which is not true (at least not for virtio-balloon). However this allows
>> me to detect if all pages in a section are offline by looking at
>> (PG_reserved && PG_offline). So I can actually tell if a page is marked
>> as offline and allocated or really offline.
>>
>>
>> 1. The location (not the number!) of PG_hwpoison is basically ABI and
>> cannot be changed. Moving it around will most probably break dump tools.
>> (see kernel/crash_core.c)
> 
> It's not ABI.  It already changed after 4.9 when PG_waiters was introduced
> by commit 62906027091f.

It is, please have a look at the file I pointed you to.

We export the *value* of PG_hwpoison in the ELF file, therefore the
*value* can change, but the *location* (page_flags, mapcount, whatever)
must not change. Or am I missing something here? I don't think we can
move PG_hwpoison that easily.

Also, I can read "For pages that are never mapped to userspace,
page->mapcount may be used for storing extra information about page
type" - is that true for PG_hwpoison/PG_reserved? I am skeptical.

And we need something similar for PG_offline, because it will become
ABI. (I can see that PAGE_BUDDY_MAPCOUNT_VALUE is also exported in an
ELF file, so maybe a new page type might work for marking a page offline
- but I have to look at the details first tomorrow)

> 
>> 2. Exposing PG_offline via kdump will make it ABI as well. And we don't
>> want any complicated validity checks ("is the bit valid or not?"),
>> because that would imply having to make these bits ABI as well. So
>> having PG_offline just like PG_hwpoison part of page_flags is the right
>> thing to do. (see patch nr 4)
>>
>> 3. For determining if all pages of a section are offline (see patch nr
>> 5), I will have to be able to check 1. PG_offline and 2. PG_reserved on
>> any page. Will this be possible by moving e.g. PG_reserved to page
>> types? (especially if some field is suddenly aliased?)
> 
> It's possible to tell whether the field is in use as mapcount or
> page_types; mapcount should always be non-negative, and page_types
> reserves a few bits to detect under/overflow of mapcount.  The slab/slob
> users of the field will also be positive uses.
> 

Thanks for the info!

-- 

Thanks,

David / dhildenb