Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

[Muhammad Usama Anjum]

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