From: Peter Xu <peterx@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>,
Juan Quintela <quintela@redhat.com>,
Pankaj Gupta <pankaj.gupta@cloud.ionos.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
teawater <teawaterz@linux.alibaba.com>,
qemu-devel@nongnu.org,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
Alex Williamson <alex.williamson@redhat.com>,
Marek Kedzierski <mkedzier@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>,
Wei Yang <richard.weiyang@linux.alibaba.com>
Subject: Re: [PATCH v3 4/7] migration/ram: Handle RAMBlocks with a RamDiscardManager on the migration source
Date: Wed, 4 Aug 2021 20:06:05 -0400 [thread overview]
Message-ID: <YQsrba6ipWwxA8au@t490s> (raw)
In-Reply-To: <20210730085249.8246-5-david@redhat.com>
On Fri, Jul 30, 2021 at 10:52:46AM +0200, David Hildenbrand wrote:
> We don't want to migrate memory that corresponds to discarded ranges as
> managed by a RamDiscardManager responsible for the mapped memory region of
> the RAMBlock. The content of these pages is essentially stale and
> without any guarantees for the VM ("logically unplugged").
>
> Depending on the underlying memory type, even reading memory might populate
> memory on the source, resulting in an undesired memory consumption. Of
> course, on the destination, even writing a zeropage consumes memory,
> which we also want to avoid (similar to free page hinting).
>
> Currently, virtio-mem tries achieving that goal (not migrating "unplugged"
> memory that was discarded) by going via qemu_guest_free_page_hint() - but
> it's hackish and incomplete.
>
> For example, background snapshots still end up reading all memory, as
> they don't do bitmap syncs. Postcopy recovery code will re-add
> previously cleared bits to the dirty bitmap and migrate them.
>
> Let's consult the RamDiscardManager after setting up our dirty bitmap
> initially and when postcopy recovery code reinitializes it: clear
> corresponding bits in the dirty bitmaps (e.g., of the RAMBlock and inside
> KVM). It's important to fixup the dirty bitmap *after* our initial bitmap
> sync, such that the corresponding dirty bits in KVM are actually cleared.
>
> As colo is incompatible with discarding of RAM and inhibits it, we don't
> have to bother.
>
> Note: if a misbehaving guest would use discarded ranges after migration
> started we would still migrate that memory: however, then we already
> populated that memory on the migration source.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
--
Peter Xu
next prev parent reply other threads:[~2021-08-05 0:07 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-30 8:52 [PATCH v3 0/7] migration/ram: Optimize for virtio-mem via RamDiscardManager David Hildenbrand
2021-07-30 8:52 ` [PATCH v3 1/7] memory: Introduce replay_discarded callback for RamDiscardManager David Hildenbrand
2021-07-30 8:52 ` [PATCH v3 2/7] virtio-mem: Implement replay_discarded RamDiscardManager callback David Hildenbrand
2021-07-30 8:52 ` [PATCH v3 3/7] migration/ram: Don't passs RAMState to migration_clear_memory_region_dirty_bitmap_*() David Hildenbrand
2021-08-05 0:05 ` Peter Xu
2021-08-05 7:41 ` Philippe Mathieu-Daudé
2021-07-30 8:52 ` [PATCH v3 4/7] migration/ram: Handle RAMBlocks with a RamDiscardManager on the migration source David Hildenbrand
2021-08-05 0:06 ` Peter Xu [this message]
2021-07-30 8:52 ` [PATCH v3 5/7] virtio-mem: Drop precopy notifier David Hildenbrand
2021-07-30 8:52 ` [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination David Hildenbrand
2021-08-05 0:04 ` Peter Xu
2021-08-05 8:10 ` David Hildenbrand
2021-08-05 12:52 ` Peter Xu
2021-08-05 7:48 ` Philippe Mathieu-Daudé
2021-08-05 8:07 ` David Hildenbrand
2021-08-05 8:17 ` Philippe Mathieu-Daudé
2021-08-05 8:20 ` David Hildenbrand
2021-07-30 8:52 ` [PATCH v3 7/7] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots David Hildenbrand
2021-08-05 8:04 ` Philippe Mathieu-Daudé
2021-08-05 8:11 ` David Hildenbrand
2021-08-05 8:21 ` Philippe Mathieu-Daudé
2021-08-05 8:27 ` David Hildenbrand
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