Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

[David Hildenbrand <david@redhat.com>]

On 02.03.23 18:19, Muhammad Usama Anjum wrote:
> On 2/28/23 5:36 AM, Peter Xu wrote:
>> On Mon, Feb 27, 2023 at 06:00:44PM -0500, Peter Xu wrote:
>>> This is a new feature that controls how uffd-wp handles none ptes.  When
>>> it's set, the kernel will handle anonymous memory the same way as file
>>> memory, by allowing the user to wr-protect unpopulated ptes.
>>>
>>> File memories handles none ptes consistently by allowing wr-protecting of
>>> none ptes because of the unawareness of page cache being exist or not.  For
>>> anonymous it was not as persistent because we used to assume that we don't
>>> need protections on none ptes or known zero pages.
>>>
>>> One use case of such a feature bit was VM live snapshot, where if without
>>> wr-protecting empty ptes the snapshot can contain random rubbish in the
>>> holes of the anonymous memory, which can cause misbehave of the guest when
>>> the guest OS assumes the pages should be all zeros.
>>>
>>> QEMU worked it around by pre-populate the section with reads to fill in
>>> zero page entries before starting the whole snapshot process [1].
>>>
>>> Recently there's another need raised on using userfaultfd wr-protect for
>>> detecting dirty pages (to replace soft-dirty in some cases) [2].  In that
>>> case if without being able to wr-protect none ptes by default, the dirty
>>> info can get lost, since we cannot treat every none pte to be dirty (the
>>> current design is identify a page dirty based on uffd-wp bit being cleared).
>>>
>>> In general, we want to be able to wr-protect empty ptes too even for
>>> anonymous.
>>>
>>> This patch implements UFFD_FEATURE_WP_UNPOPULATED so that it'll make
>>> uffd-wp handling on none ptes being consistent no matter what the memory
>>> type is underneath.  It doesn't have any impact on file memories so far
>>> because we already have pte markers taking care of that.  So it only
>>> affects anonymous.
>>>
>>> The feature bit is by default off, so the old behavior will be maintained.
>>> Sometimes it may be wanted because the wr-protect of none ptes will contain
>>> overheads not only during UFFDIO_WRITEPROTECT (by applying pte markers to
>>> anonymous), but also on creating the pgtables to store the pte markers. So
>>> there's potentially less chance of using thp on the first fault for a none
>>> pmd or larger than a pmd.
>>>
>>> The major implementation part is teaching the whole kernel to understand
>>> pte markers even for anonymously mapped ranges, meanwhile allowing the
>>> UFFDIO_WRITEPROTECT ioctl to apply pte markers for anonymous too when the
>>> new feature bit is set.
>>>
>>> Note that even if the patch subject starts with mm/uffd, there're a few
>>> small refactors to major mm path of handling anonymous page faults. But
>>> they should be straightforward.
>>>
>>> So far, add a very light smoke test within the userfaultfd kselftest
>>> pagemap unit test to make sure anon pte markers work.
>>>
>>> [1] https://lore.kernel.org/all/20210401092226.102804-4-andrey.gruzdev@virtuozzo.com/
>>> [1] https://lore.kernel.org/all/Y+v2HJ8+3i%2FKzDBu@x1n/
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>> v1->v2:
>>> - Use pte markers rather than populate zero pages when protect [David]
>>> - Rename WP_ZEROPAGE to WP_UNPOPULATED [David]
>>
>> Some very initial performance numbers (I only ran in a VM but it should be
>> similar, unit is "us") below as requested.  The measurement is about time
>> spent when wr-protecting 10G range of empty but mapped memory.  It's done
>> in a VM, assuming we'll get similar results on bare metal.
>>
>> Four test cases:
>>
>>          - default UFFDIO_WP
>>          - pre-read the memory, then UFFDIO_WP (what QEMU does right now)
>>          - pre-fault using MADV_POPULATE_READ, then default UFFDIO_WP
>>          - UFFDIO_WP with WP_UNPOPULATED
>>
>> Results:
>>
>>          Test DEFAULT: 2
>>          Test PRE-READ: 3277099 (pre-fault 3253826)
>>          Test MADVISE: 2250361 (pre-fault 2226310)
>>          Test WP-UNPOPULATE: 20850
> In your case:
> Default < WP-UNPOPULATE < MADVISE < PRE-READ
> 
> 
> In my testing on next-20230228 with this patch and my uffd async patch:
> 
> Test DEFAULT: 6
> Test PRE-READ: 37157 (pre-fault 37006)
> Test MADVISE: 4884 (pre-fault 4465)
> Test WP-UNPOPULATE: 17794
> 
> DEFAULT < MADVISE < WP-UNPOPULATE < PRE-READ
> 
> On my setup, MADVISE is performing better than WP-UNPOPULATE consistently.
> I'm not sure why I'm getting this discrepancy here. I've liked your results
> to be honest where we perform better with WP-UNPOPULATE than MADVISE. What
> can be done to get consistent benchmarks over your and my side?

Probably because the current approach from Peter uses uffd-wp markers, 
and these markers can currently only reside on the PTE level, not on the 
PMD level yet.

With MADVISE you get a huge zeropage and avoid dealing with PTEs.

-- 
Thanks,

David / dhildenb