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

[mdroth <mdroth@linux.vnet.ibm.com>]

On Mon, Jun 10, 2013 at 12:14:20PM +0200, Peter Lieven wrote:
> 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 <pl@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)) {
> +                memset(host, ch, TARGET_PAGE_SIZE);
>  #ifndef _WIN32
> -            if (ch == 0 &&
> -                (!kvm_enabled() || kvm_has_sync_mmu()) &&
> -                getpagesize() <= TARGET_PAGE_SIZE) {
> -                qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
> -            }
> +                if (ch == 0 &&
> +                    (!kvm_enabled() || kvm_has_sync_mmu()) &&
> +                    getpagesize() <= TARGET_PAGE_SIZE) {
> +                    qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
> +                }

Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>

Also CC'ing qemu-stable, but from what I gather this just mitigates the
performance impact of 1/2 and isn't strictly necessary to fix migration?
i.e. we can still do outgoing migration to 1.5.0 guests?

>  #endif
> +            }
>          } else if (flags & RAM_SAVE_FLAG_PAGE) {
>              void *host;
> 
> -- 
> 1.7.9.5
> 
>