Hi Peter, Finally I've found the bug. On 2/28/23 9:24 PM, Muhammad Usama Anjum wrote: > 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'; While using WP_UNPOPULATED, we get stuck if newly allocated memory is read without initialization. This can be reproduced by either of the following statements: printf("%c", buffer[0]); buffer[0]++; This bug has start to appear on this patch. How are you handling reading newly allocated memory when WP_UNPOPULATED is defined? Running my pagemap_ioctl selftest as benchmark in a VM: without zeropage / wp_unpopulated (decide from pte_none() if page is dirty or not, buggy and wrong implementation, just for reference) 26.608 seconds with zeropage 39.203 seconds with wp_unpopulated 62.907 seconds 136% worse performance overall 60% worse performance of unpopulated than zeropage >>> 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
