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SubjectRe: [PATCH RFC 2/8] mm: introduce PG_offline
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.

> 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.

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