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SubjectRe: [PATCH RFC 2/8] mm: introduce PG_offline
On Sun 22-04-18 17:13:52, David Hildenbrand wrote:
> 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)

Wait wait wait. Who is relying on this? Kdump? Page flags have always
been an internal implementation detail and _nobody_ outside of the
kernel should ever rely on the specific value. Well, kdump has been
cheating but that is because kdump is inherently tight to a specific
kernel implementation but that doesn't make it a stable ABI IMHO.
Restricting the kernel internals because of a debugging tool would be
quite insane.
Michal Hocko

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