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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 18:15:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 29.07.21 17:52, Peter Xu wrote:
On Thu, Jul 29, 2021 at 02:14:41PM +0200, David Hildenbrand wrote:
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.

I'm wondering whether we should make the offset page size aligned instead.  For
example, note that postcopy_place_page_zero() should only take page_size
aligned host addr or UFFDIO_COPY could fail (hugetlb doesn't support

That is true indeed. I'd assume in that case that we would get called with the proper offset already, right? Because uffd would only report properly aligned pages IIRC.

Btw, does virtio-mem supports hugetlbfs now?  When with it, the smallest unit
to plug/unplug would the huge page size (e.g., for 1g huge page, sounds not
helpful to unplug 2M memory), am I right?

It supports it to 99.9 % I'd say (especially with the dump/tpm/migration fixes upstream). The units are handled properly: the unit is at least as big as the page size.

So with 1 GiB pages, you have a unit of 1 GiB.


David / dhildenb

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