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Re: [PATCH v2 6/6] migration/ram: Handle RAMBlocks with a RamDiscardMana

From: Peter Xu
Subject: Re: [PATCH v2 6/6] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots
Date: Fri, 23 Jul 2021 12:37:47 -0400

On Wed, Jul 21, 2021 at 11:27:59AM +0200, David Hildenbrand wrote:
> We already don't ever migrate memory that corresponds to discarded ranges
> as managed by a RamDiscardManager responsible for the mapped memory region
> of the RAMBlock.
> virtio-mem uses this mechanism to logically unplug parts of a RAMBlock.
> Right now, we still populate zeropages for the whole usable part of the
> RAMBlock, which is undesired because:
> 1. Even populating the shared zeropage will result in memory getting
>    consumed for page tables.
> 2. Memory backends without a shared zeropage (like hugetlbfs and shmem)
>    will populate an actual, fresh page, resulting in an unintended
>    memory consumption.
> Discarded ("logically unplugged") parts have to remain discarded. As
> these pages are never part of the migration stream, there is no need to
> track modifications via userfaultfd WP reliably for these parts.
> Further, any writes to these ranges by the VM are invalid and the
> behavior is undefined.
> Note that Linux only supports userfaultfd WP on private anonymous memory
> for now, which usually results in the shared zeropage getting populated.
> The issue will become more relevant once userfaultfd WP supports shmem
> and hugetlb.
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Peter Xu <peterx@redhat.com>

Peter Xu

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