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Date: Tue, 04 Jun 2013 16:14:15 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] broken incoming migration
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To: Peter Lieven <pl@kamp.de>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>, "qemu-ppc@nongnu.org" <qemu-ppc@nongnu.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, David Gibson <david@gibson.dropbear.id.au>

Il 04/06/2013 15:52, Peter Lieven ha scritto:
> On 30.05.2013 16:41, Paolo Bonzini wrote:
>> Il 30/05/2013 16:38, Peter Lieven ha scritto:
>>>>> You could also scan the page for nonzero values before writing it.
>>> i had this in mind, but then choosed the other approach.... turned
>>> out to be a bad idea.
>>>
>>> alexey: i will prepare a patch later today, could you then please
>>> verify it fixes your problem.
>>>
>>> paolo: would we still need the madvise or is it enough to not write
>>> the zeroes?
>> It should be enough to not write them.
> 
> Problem: checking the pages for zero allocates them. even at the source.

It doesn't look like.  I tried this program and top doesn't show an
increasing amount of reserved memory:

#include <stdio.h>
#include <stdlib.h>
int main()
{
    char *x = malloc(500 << 20);
    int i, j;
    for (i = 0; i < 500; i += 10) {
        for (j = 0; j < 10 << 20; j += 4096) {
             *(volatile char*) (x + (i << 20) + j);
        }
        getchar();
    }
}

Paolo