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Re: [PATCH v2 5/6] migration/postcopy: Handle RAMBlocks with a RamDiscar

From: David Hildenbrand
Subject: Re: [PATCH v2 5/6] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination
Date: Thu, 29 Jul 2021 14:14:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 24.07.21 00:10, Peter Xu wrote:
On Fri, Jul 23, 2021 at 09:01:42PM +0200, David Hildenbrand wrote:
It can happen in corner cases and is valid: with the current virtio-mem
spec, guests are allowed to read unplugged memory. This will, for example,
happen on older Linux guests when reading /proc/kcore or (with even older
guests) when dumping guest memory via kdump. These corner cases were the
main reason why the spec allows for it -- until we have guests properly
adjusted such that it won't happen even in corner cases.

A future feature bit will disallow it for the guest: required for supporting
shmem/hugetlb cleanly. With that in place, I agree that we would want to
warn in this case!

OK that makes sense; with the page_size change, feel free to add:

I just realized that relying on the page_size would be wrong.

We migrate TARGET_PAGE_SIZE chunks and the offset might not be page_size aligned. So if we were to replace TARGET_PAGE_SIZE by rb->page_size, we might accidentally cover a "too big" range.

Makes sense?

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


David / dhildenb

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