Re: [Qemu-devel] [PATCH v2 00/16] Postcopy: Hugepage support

[Andrea Arcangeli <aarcange@redhat.com>]

Hello Alexey,

On Tue, Feb 14, 2017 at 05:48:25PM +0300, Alexey Perevalov wrote:
> On Mon, Feb 13, 2017 at 06:57:22PM +0100, Andrea Arcangeli wrote:
> > Hello,
> > 
> > On Mon, Feb 13, 2017 at 08:11:06PM +0300, Alexey Perevalov wrote:
> > > Another one request.
> > > QEMU could use mem_path in hugefs with share key simultaneously
> > > (-object memory-backend-file,id=mem,size=${mem_size},mem-path=${mem_path},share=on) and vm
> > > in this case will start and will properly work (it will allocate memory
> > > with mmap), but in case of destination for postcopy live migration
> > > UFFDIO_COPY ioctl will fail for
> > > such region, in Arcangeli's git tree there is such prevent check
> > > (if (!vma_is_shmem(dst_vma) && dst_vma->vm_flags & VM_SHARED).
> > > Is it possible to handle such situation at qemu?
> > 
> > It'd be nice to lift this hugetlbfs !VM_SHARED restriction I agree, I
> > already asked Mike (CC'ed) why is there, because I'm afraid it's a
> > leftover from the anon version where VM_SHARED means a very different
> > thing but it was already lifted for shmem. share=on should already
> > work on top of tmpfs and also with THP on tmpfs enabled.
> > 
> > For hugetlbfs and shmem it should be generally more complicated to
> > cope with private mappings than shared ones, shared is just the native
> > form of the pseudofs without having to deal with private COWs aliases
> > so it's hard to imagine something going wrong for VM_SHARED if the
> > MAP_PRIVATE mapping already works fine. If it turns out to be
> > superflous the check may be just turned into
> > "vma_is_anonymous(dst_vma) && dst_vma->vm_flags & VM_SHARED".
> 
> Great, as I know  -netdev type=vhost-user requires share=on in
> -object memory-backend in ovs-dpdk scenario
> http://wiki.qemu-project.org/Documentation/vhost-user-ovs-dpdk

share=on should work now with current aa.git userfault branch, and the
support is already included in -mm, it should all get merged upstream
in kernel 4.11.

Could you test the current aa.git userfault branch to verify postcopy
live migration works fine on hugetlbfs share=on?

Thanks!
Andrea