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

[Andrea Arcangeli]

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

Cc'ed not existent email, mail client autocompletion error, corrected
the CC.

> 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".
> 
> Thanks,
> Andrea