Re: [Qemu-devel] [PATCHv2 2/2] migration: do not overwrite zero pages

[Peter Lieven]

On 13.06.2013 05:30, Wenchao Xia wrote:
> 于 2013-6-10 18:14, Peter Lieven 写道:
>> on incoming migration do not memset pages to zero if they already read as 
>> zero.
>> this will allocate a new zero page and consume memory unnecessarily. even
>> if we madvise a MADV_DONTNEED later this will only deallocate the memory
>> asynchronously.
>>
>> Signed-off-by: Peter Lieven <p...@kamp.de>
>> ---
>>   arch_init.c |   14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch_init.c b/arch_init.c
>> index 08fccf6..cf4e1d5 100644
>> --- a/arch_init.c
>> +++ b/arch_init.c
>> @@ -832,14 +832,16 @@ static int ram_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>               }
>>
>>               ch = qemu_get_byte(f);
>> -            memset(host, ch, TARGET_PAGE_SIZE);
>> +            if (ch != 0 || !is_zero_page(host)) {
>   If incoming page is not zero, always memset. If incoming page is
> zero, then if on destination it is not zero, memset. Logic is OK.
> Only question is if the read operation in is_zero_page() consumes
> memory, as there are doubt in the discuss before.

It does not consume memory for normal pages and for thp since 3.8.

BR,
Peter