Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

[David Hildenbrand]

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@xxxxxxxxxxxxx/
> > > [1] https://lore.kernel.org/all/Y+v2HJ8+3i%2FKzDBu@x1n/
> > >
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > > 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