Re: [Qemu-devel] [PATCH v2 09/16] postcopy: Use temporary for placing zero huge pages

[Laurent Vivier]

On 24/02/2017 16:46, Dr. David Alan Gilbert wrote:
> * Laurent Vivier (address@hidden) wrote:
>> On 06/02/2017 18:32, Dr. David Alan Gilbert (git) wrote:
>>> From: "Dr. David Alan Gilbert" <address@hidden>
>>>
>>> The kernel can't do UFFDIO_ZEROPAGE for huge pages, so we have
>>> to allocate a temporary (always zero) page and use UFFDIO_COPYPAGE
>>> on it.
>>>
>>> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
>>> Reviewed-by: Juan Quintela <address@hidden>
>>> ---
...
>> Are you sure the kernel doesn't support UFFDIO_ZEROPAGE for huge page.
>> It seems __mcopy_atomic() manages HUGETLB vma (it is called by
>> mfill_zeropage(), called by userfaultfd_zeropage())?
> 
> That's as I understand it from Andrea; and I think it does fail if you try it.

Found the answer in kernel log:

    commit 7a0c4cf85b856430af62a907dd65dfc51438d24f
    Author: Andrea Arcangeli <address@hidden>
    Date:   Wed Feb 22 15:44:10 2017 -0800

        userfaultfd: selftest: test UFFDIO_ZEROPAGE on all memory types

        This will verify -EINVAL is returned with hugetlbfs/shmem and
        it'll do a functional test of UFFDIO_ZEROPAGE on anonymous
        memory.

Thanks,
Laurent