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Postcopy with different page sizes on source and destination
From: |
Florian Schmidt |
Subject: |
Postcopy with different page sizes on source and destination |
Date: |
Mon, 20 Apr 2020 09:14:01 +0000 |
Hi,
with precopy live migration, change in page size on source and
destination is possible: using hugetlbfs memory backing for the VM on
the source and anonymous memory on the destination, and vice versa. For
postcopy migration, this is not allowed, and in fact checked during the
advise stage.
Is there any fundamental limitation in the design that prevents this, or
is it more that this is an additional complication that nobody has
implemented so far because there was no strong need for it?
It seems to me like this should be possible, and the comment in
loadvm_postcopy_handle_advise() (migration/savevm.c:1681) also seems to
suggest that; so I'll add a (very rough) first idea. Please tell me if
I'm missing something important. The "background" copy is similar to
precopy, so the main difference is the userfaultfd page fault handling
on the destination, and requesting the correct memory from the source.
1. If the source has hugepages and the destination doesn't, then a page
fault would lead the destination to ask "I need these 4k of memory from
you to fill my page and handle the page fault". The source could then
answer "here you are, and here are these other 511 4k pages around it
(which form my 2M page; similarly for 1G pages), please deal with them
now". That way, even "release-ram" would still work on a (huge)page
granularity.
2. If the destination has hugepages and the source doesn't, then the
above works similarly: now the destination, on a page fault, asks for a
larger memory area that corresponds to 512 (or more) pages on the
source. The only issue I could see here is during the initial phase,
when postcopy is switched on, to make sure that the source doesn't
release RAM that it has copied and thinks is clean, but it part of a
hugepage on the other side. That seems easy enough to solve though? And
indeed is probably already implemented for precopy migration to work
with different page sizes on source and destination and could be adapted
here.
Cheers,
Florian
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