Re: [RFC PATCH 0/2] userfaultfd: handle minor faults, add UFFDIO_CONTINUE

[Mike Kravetz]

On 1/11/21 3:08 PM, Peter Xu wrote:
> On Mon, Jan 11, 2021 at 02:42:48PM -0800, Mike Kravetz wrote:
>> On 1/7/21 11:04 AM, Axel Rasmussen wrote:
>>> Overview
>>> ========
>>>
>>> This series adds a new userfaultfd registration mode,
>>> UFFDIO_REGISTER_MODE_MINOR. This allows userspace to intercept "minor" faults.
>>> By "minor" fault, I mean the following situation:
>>>
>>> Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory).
>>> One of the mappings is registered with userfaultfd (in minor mode), and the
>>> other is not. Via the non-UFFD mapping, the underlying pages have already been
>>> allocated & filled with some contents. The UFFD mapping has not yet been
>>> faulted in; when it is touched for the first time, this results in what I'm
>>> calling a "minor" fault. As a concrete example, when working with hugetlbfs, we
>>> have huge_pte_none(), but find_lock_page() finds an existing page.
>>>
>>> We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is,
>>> userspace resolves the fault by either a) doing nothing if the contents are
>>> already correct, or b) updating the underlying contents using the second,
>>> non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA,
>>> or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel
>>> "I have ensured the page contents are correct, carry on setting up the mapping".
>>>
>>
>> One quick thought.
>>
>> This is not going to work as expected with hugetlbfs pmd sharing.  If you
>> are not familiar with hugetlbfs pmd sharing, you are not alone. :)
>>
>> pmd sharing is enabled for x86 and arm64 architectures.  If there are multiple
>> shared mappings of the same underlying hugetlbfs file or shared memory segment
>> that are 'suitably aligned', then the PMD pages associated with those regions
>> are shared by all the mappings.  Suitably aligned means 'on a 1GB boundary'
>> and 1GB in size.
>>
>> When pmds are shared, your mappings will never see a 'minor fault'.  This
>> is because the PMD (page table entries) is shared.
> 
> Thanks for raising this, Mike.
> 
> I've got a few patches that plan to disable huge pmd sharing for uffd in
> general, e.g.:
> 
> https://github.com/xzpeter/linux/commit/f9123e803d9bdd91bf6ef23b028087676bed1540
> https://github.com/xzpeter/linux/commit/aa9aeb5c4222a2fdb48793cdbc22902288454a31
> 
> I believe we don't want that for missing mode too, but it's just not extremely
> important for missing mode yet, because in missing mode we normally monitor all
> the processes that will be using the registered mm range.  For example, in QEMU
> postcopy migration with vhost-user hugetlbfs files as backends, we'll monitor
> both the QEMU process and the DPDK program, so that either of the programs will
> trigger a missing fault even if pmd shared between them.  However again I think
> it's not ideal since uffd (even if missing mode) is pgtable-based, so sharing
> could always be too tricky.
> 
> They're not yet posted to public yet since that's part of uffd-wp support for
> hugetlbfs (along with shmem).  So just raise this up to avoid potential
> duplicated work before I post the patchset.
> 
> (Will read into details soon; probably too many things piled up...)

Thanks for the heads up about this Peter.

I know Oracle DB really wants shared pmds -and- UFFD.  I need to get details
of their exact usage model.  I know they primarily use SIGBUS, but use
MISSING_HUGETLBFS as well.  We may need to be more selective in when to
disable.

-- 
Mike Kravetz