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    SubjectRe: [PATCH RFC 2/8] mm: introduce PG_offline
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    On 13.04.2018 15:40, Michal Hocko wrote:
    > On Fri 13-04-18 15:16:26, David Hildenbrand wrote:
    >> online_pages()/offline_pages() theoretically allows us to work on
    >> sub-section sizes. This is especially relevant in the context of
    >> virtualization. It e.g. allows us to add/remove memory to Linux in a VM in
    >> 4MB chunks.
    >
    > Well, theoretically possible but this would require a lot of auditing
    > because the hotplug and per section assumption is quite a spread one.

    Indeed. But besides changing section sizes / size of memory blocks this
    seems to be the only way to do it. (btw, I think Windows allows to add
    1MB chunks - e.g. 1MB DIMMs)

    But as these pages "belong to nobody" nobody (besides kdump) should dare
    to access the content, although the section is online.

    >
    >> While the whole section is marked as online/offline, we have to know
    >> the state of each page. E.g. to not read memory that is not online
    >> during kexec() or to properly mark a section as offline as soon as all
    >> contained pages are offline.
    >
    > But you cannot use a page flag for that, I am afraid. Page flags are
    > extremely scarce resource. I haven't looked at the rest of the series
    > but _if_ we have a bit spare which I am not really sure about then you
    > should prove there are no other ways around this.

    Open for suggestions. We could remember per segment/memory block which
    parts are online/offline and use that to decide if a section can go offline.

    However: kdump will also have to (easily) know which pages are offline,
    so it can skip reading them. (see the other patch)

    >
    >> Signed-off-by: David Hildenbrand <david@redhat.com>


    --

    Thanks,

    David / dhildenb

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