On 2/28/23 8:58 PM, Peter Xu wrote: > On Tue, Feb 28, 2023 at 12:21:45PM +0500, Muhammad Usama Anjum wrote: >> Hi Peter, > > Hi, Muhammad, > >> >> Thank you so much for sending. >> >> 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 >>> >>> I'll add these information into the commit message when there's a new >>> version. >> I'm hitting a bug where I'm unable to write to the memory after adding this >> patch and wp the memory. I'm hitting this case in your test and my tests as >> well. Please apply the following diff to your test to reproduce on your end: >> >> --- uffd_wp_perf.c.orig 2023-02-28 12:09:38.971820791 +0500 >> +++ uffd_wp_perf.c 2023-02-28 12:13:11.077827160 +0500 >> @@ -114,6 +114,7 @@ >> start1 = get_usec(); >> } >> wp_range(uffd, buffer, SIZE, true); >> + buffer[0] = 'a'; >> if (start1 == 0) >> printf("%"PRIu64"\n", get_usec() - start); >> else > > This is expected, because the test didn't start any fault resolving thread, > so the write will block until someone unprotects the page. Ohh.. sorry. Wrong reproducer. > > But it shouldn't happen to your use case if you applied both WP_UNPOPULATED > & WP_ASYNC. I'm using both WP_UNPOPULATED and ASYNC. The program gets stuck at right time: 1..57 ok 1 sanity_tests_sd wrong flag specified ok 2 sanity_tests_sd wrong mask specified ok 3 sanity_tests_sd wrong return mask specified ok 4 sanity_tests_sd mixture of correct and wrong flag ok 5 sanity_tests_sd PM_SCAN_OP_WP cannot be used without get ok 6 sanity_tests_sd Clear area with larger vec size ^C Program received signal SIGINT, Interrupt. 0x000000000040220c in sanity_tests_sd () at pagemap_ioctl.c:198 198 mem[i]++; (gdb) bt #0 0x000000000040220c in sanity_tests_sd () at pagemap_ioctl.c:198 #1 0x0000000000404e14 in main () at pagemap_ioctl.c:846 () /proc/$PID/stack is empty. Not sure why. I can see stack trace of other applications, but not this one's. Let me send better reproducer for you. > > Could you try "cat /proc/$PID/stack" to see where does your thread stuck > at? > -- BR, Muhammad Usama Anjum
