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

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

On Mon, Jun 10, 2013 at 11:10:29AM -0500, mdroth wrote:
> 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

*actually* CC'ing qemu-stable this time :)