Messages in this thread | | | Date | Thu, 2 Mar 2023 18:38:20 +0100 | Subject | Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED | From | David Hildenbrand <> |
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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
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