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

[Mike Kravetz]

On 02/13/2017 10:10 AM, Andrea Arcangeli 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
> 
> 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

Sorry, I did not see e-mail earlier.

Andrea is correct in that the VM_SHARED restriction for hugetlbfs was there
to make the code common with the anon version.  The use case I had was to
simply 'catch' no page hugetlbfs faults private -or- shared.  That is why
you can register hugetlbfs shared regions.

I can take a look at what it would take to enable copy, and agree with Andrea
that it should be relatively easy.

-- 
Mike Kravetz