Re: [Qemu-devel] [RFC 23/29] vub+postcopy: madvises

[Alexey Perevalov]

On 08/08/2017 08:06 PM, Dr. David Alan Gilbert wrote:
> * Alexey Perevalov (address@hidden) wrote:
> > On 06/28/2017 10:00 PM, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <address@hidden>
> > >
> > > Clear the area and turn off THP.
> > >
> > > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > > ---
> > >    contrib/libvhost-user/libvhost-user.c | 32 ++++++++++++++++++++++++++++++--
> > >    1 file changed, 30 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/contrib/libvhost-user/libvhost-user.c 
> > > b/contrib/libvhost-user/libvhost-user.c
> > > index 0658b6e847..ceddeac74f 100644
> > > --- a/contrib/libvhost-user/libvhost-user.c
> > > +++ b/contrib/libvhost-user/libvhost-user.c
> > > @@ -451,11 +451,39 @@ vu_set_mem_table_exec(VuDev *dev, VhostUserMsg *vmsg)
> > >            }
> > >            if (dev->postcopy_listening) {
> > > +            int ret;
> > >                /* We should already have an open ufd need to mark each memory
> > >                 * range as ufd.
> > > -             * Note: Do we need any madvises? Well it's not been accessed
> > > -             * yet, still probably need no THP to be safe, discard to be safe?
> > >                 */
> > > +
> > > +            /* Discard any mapping we have here; note I can't use MADV_REMOVE
> > > +             * or fallocate to make the hole since I don't want to lose
> > > +             * data that's already arrived in the shared process.
> > > +             * TODO: How to do hugepage
> > > +             */
> > Hi, David, frankly saying, I stuck with my solution, and I have also another
> > issues,
> > but here I could suggest solution for hugepages. I think we could transmit a
> > received pages
> > bitmap in VHOST_USER_SET_MEM_TABLE (VhostUserMemoryRegion), but it will
> > raise a compatibility issue,
> > or introduce special message type for that and send it before
> > VHOST_USER_SET_MEM_TABLE.
> > So it will be  possible to do fallocate on received bitmap basis, just skip
> > already copied pages.
> > If you wish, I could send patches, rebased on yours, for doing it.
> What we found works is that actually we don't need to do a discard -
> since we've only just done the mmap of the arena, nothing will be
> occupying it on the shared client, so we don't need to discard.
Looks like yes, I checked on kernel from Andrea's git,
there is any more EEXIST error in case when client doesn't
fallocate.

> We've had a postcopy migrate work now, with a few hacks we're still
> cleaning up, both on vhost-user-bridge and dpdk; so I'll get this
> updated and reposted.
In you patch series vring is disabling in case of VHOST_USER_GET_VRING_BASE.
It's being called when vhost-user server want's to stop vring.
QEMU is enabling vring as soon as virtual machine is started, so I 
didn't see
explicit vring disabling for migrating VRING.
So migrating VRING is protected just by uffd_register, isn't it? And PMD 
thread (any
vhost-user thread which accessing migrating VRING) will wait page 
copying in this case,
right?

> Dave
>
> > > +            ret = madvise((void *)dev_region->mmap_addr,
> > > +                          dev_region->size + dev_region->mmap_offset,
> > > +                          MADV_DONTNEED);
> > > +            if (ret) {
> > > +                fprintf(stderr,
> > > +                        "%s: Failed to madvise(DONTNEED) region %d: %s\n",
> > > +                        __func__, i, strerror(errno));
> > > +            }
> > > +            /* Turn off transparent hugepages so we dont get lose wakeups
> > > +             * in neighbouring pages.
> > > +             * TODO: Turn this backon later.
> > > +             */
> > > +            ret = madvise((void *)dev_region->mmap_addr,
> > > +                          dev_region->size + dev_region->mmap_offset,
> > > +                          MADV_NOHUGEPAGE);
> > > +            if (ret) {
> > > +                /* Note: This can happen legally on kernels that are configured
> > > +                 * without madvise'able hugepages
> > > +                 */
> > > +                fprintf(stderr,
> > > +                        "%s: Failed to madvise(NOHUGEPAGE) region %d: %s\n",
> > > +                        __func__, i, strerror(errno));
> > > +            }
> > >                struct uffdio_register reg_struct;
> > >                /* Note: We might need to go back to using mmap_addr and
> > >                 * len + mmap_offset for * huge pages, but then we do hope not to
> >
> > --
> > Best regards,
> > Alexey Perevalov
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK

--
Best regards,
Alexey Perevalov