Re: [Qemu-devel] [PATCH] migration: Fix madvise breakage if host and guest have different page sizes

[David Gibson <david@gibson.dropbear.id.au>]

On Wed, Nov 21, 2012 at 03:59:25PM +1100, David Gibson wrote:
> madvise(DONTNEED) will throw away the contents of the whole page at the
> given address, even if the given length is less than the page size.  One
> can argue about whether that's the correct behaviour, but that's what it's
> done for a long time in Linux at least.
> 
> That means that the madvise() in ram_load(), on a setup where
> TARGET_PAGE_SIZE is smaller than the host page size, can throw away data
> in guest pages adjacent to the one it's actually processing right now,
> leading to guest memory corruption on an incoming migration.
> 
> This patch therefore, disables the madvise() if the host page size is
> larger than TARGET_PAGE_SIZE.  This means we don't get the benefits of that
> madvise() in this case, but a more complete fix is more difficult to
> accomplish.  This at least fixes the guest memory corruption.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Sorry, forgot to add:

Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
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