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? > > I'll add these information into the commit message when there's a new > version. > > [1] https://github.com/xzpeter/clibs/blob/master/uffd-test/uffd-wp-perf.c > -- BR, Muhammad Usama Anjum
