From: Peter Xu <peterx@redhat.com>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Muhammad Usama Anjum <usama.anjum@collabora.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
Axel Rasmussen <axelrasmussen@google.com>,
Nadav Amit <nadav.amit@gmail.com>,
David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED
Date: Mon, 27 Feb 2023 19:36:23 -0500 [thread overview]
Message-ID: <Y/1Mh5uivFt+zWKM@x1n> (raw)
In-Reply-To: <20230227230044.1596744-1-peterx@redhat.com>
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
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
--
Peter Xu
next prev parent reply other threads:[~2023-02-28 0:37 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-27 23:00 [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED Peter Xu
2023-02-28 0:36 ` Peter Xu [this message]
2023-02-28 7:21 ` Muhammad Usama Anjum
2023-02-28 15:58 ` Peter Xu
2023-02-28 16:24 ` Muhammad Usama Anjum
2023-03-01 7:55 ` Muhammad Usama Anjum
2023-03-01 15:19 ` Peter Xu
2023-03-01 17:13 ` Muhammad Usama Anjum
2023-03-02 9:37 ` David Hildenbrand
2023-03-02 13:57 ` Peter Xu
2023-03-02 14:01 ` David Hildenbrand
2023-03-02 15:14 ` Muhammad Usama Anjum
2023-03-02 22:00 ` Peter Xu
2023-03-02 17:19 ` Muhammad Usama Anjum
2023-03-02 17:38 ` David Hildenbrand
2023-03-02 22:21 ` Peter Xu
2023-03-03 6:42 ` Muhammad Usama Anjum
2023-03-03 16:47 ` Peter Xu
2023-03-06 9:03 ` Muhammad Usama Anjum
2023-03-06 16:09 ` Muhammad Usama Anjum
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