Message-ID: <598965cb-85d3-5b33-a1d4-2f49e94ee8ea@collabora.com>
Date:   Tue, 30 May 2023 19:07:44 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        Michał Mirosław <emmir@...gle.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Andrei Vagin <avagin@...il.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH v16 0/5] Implement IOCTL to get and optionally clear info
 about PTEs

Hello Peter, David, Michal, Mike, Danylo and Andrei,

I hope you are well. You (and some other) guys have been helping and
reviewing up to this point. Thank you so much! Please review them again and
mention if anything comes to your mind. Please send review tags or tested
by tags in the hope that we merge these soon for next release. Current
patches fulfill all of our requirement regarding ABI and performance. I
guess, we should merge now (if somethings comes up, we can always fix them
along the way).

Any thoughts/comments are welcome. I'm sending this email as we have not
had much reviews in the past 3-4 revisions.

Thanks,
Usama

On 5/25/23 1:55 PM, Muhammad Usama Anjum wrote:
> *Changes in v16*
> - Fix a corner case
> - Add exclusive PM_SCAN_OP_WP back
> 
> *Changes in v15*
> - Build fix (Add missed build fix in RESEND)
> 
> *Changes in v14*
> - Fix build error caused by #ifdef added at last minute in some configs
> 
> *Changes in v13*
> - Rebase on top of next-20230414
> - Give-up on using uffd_wp_range() and write new helpers, flush tlb only
>   once
> 
> *Changes in v12*
> - Update and other memory types to UFFD_FEATURE_WP_ASYNC
> - Rebaase on top of next-20230406
> - Review updates
> 
> *Changes in v11*
> - Rebase on top of next-20230307
> - Base patches on UFFD_FEATURE_WP_UNPOPULATED
> - Do a lot of cosmetic changes and review updates
> - Remove ENGAGE_WP + !GET operation as it can be performed with
>   UFFDIO_WRITEPROTECT
> 
> *Changes in v10*
> - Add specific condition to return error if hugetlb is used with wp
>   async
> - Move changes in tools/include/uapi/linux/fs.h to separate patch
> - Add documentation
> 
> *Changes in v9:*
> - Correct fault resolution for userfaultfd wp async
> - Fix build warnings and errors which were happening on some configs
> - Simplify pagemap ioctl's code
> 
> *Changes in v8:*
> - Update uffd async wp implementation
> - Improve PAGEMAP_IOCTL implementation
> 
> *Changes in v7:*
> - Add uffd wp async
> - Update the IOCTL to use uffd under the hood instead of soft-dirty
>   flags
> 
> *Motivation*
> The real motivation for adding PAGEMAP_SCAN IOCTL is to emulate Windows
> GetWriteWatch() syscall [1]. The GetWriteWatch{} retrieves the addresses of
> the pages that are written to in a region of virtual memory.
> 
> This syscall is used in Windows applications and games etc. This syscall is
> being emulated in pretty slow manner in userspace. Our purpose is to
> enhance the kernel such that we translate it efficiently in a better way.
> Currently some out of tree hack patches are being used to efficiently
> emulate it in some kernels. We intend to replace those with these patches.
> So the whole gaming on Linux can effectively get benefit from this. It
> means there would be tons of users of this code.
> 
> CRIU use case [2] was mentioned by Andrei and Danylo:
>> Use cases for migrating sparse VMAs are binaries sanitized with ASAN,
>> MSAN or TSAN [3]. All of these sanitizers produce sparse mappings of
>> shadow memory [4]. Being able to migrate such binaries allows to highly
>> reduce the amount of work needed to identify and fix post-migration
>> crashes, which happen constantly.
> 
> Andrei's defines the following uses of this code:
> * it is more granular and allows us to track changed pages more
>   effectively. The current interface can clear dirty bits for the entire
>   process only. In addition, reading info about pages is a separate
>   operation. It means we must freeze the process to read information
>   about all its pages, reset dirty bits, only then we can start dumping
>   pages. The information about pages becomes more and more outdated,
>   while we are processing pages. The new interface solves both these
>   downsides. First, it allows us to read pte bits and clear the
>   soft-dirty bit atomically. It means that CRIU will not need to freeze
>   processes to pre-dump their memory. Second, it clears soft-dirty bits
>   for a specified region of memory. It means CRIU will have actual info
>   about pages to the moment of dumping them.
> * The new interface has to be much faster because basic page filtering
>   is happening in the kernel. With the old interface, we have to read
>   pagemap for each page.
> 
> *Implementation Evolution (Short Summary)*
> From the definition of GetWriteWatch(), we feel like kernel's soft-dirty
> feature can be used under the hood with some additions like:
> * reset soft-dirty flag for only a specific region of memory instead of
> clearing the flag for the entire process
> * get and clear soft-dirty flag for a specific region atomically
> 
> So we decided to use ioctl on pagemap file to read or/and reset soft-dirty
> flag. But using soft-dirty flag, sometimes we get extra pages which weren't
> even written. They had become soft-dirty because of VMA merging and
> VM_SOFTDIRTY flag. This breaks the definition of GetWriteWatch(). We were
> able to by-pass this short coming by ignoring VM_SOFTDIRTY until David
> reported that mprotect etc messes up the soft-dirty flag while ignoring
> VM_SOFTDIRTY [5]. This wasn't happening until [6] got introduced. We
> discussed if we can revert these patches. But we could not reach to any
> conclusion. So at this point, I made couple of tries to solve this whole
> VM_SOFTDIRTY issue by correcting the soft-dirty implementation:
> * [7] Correct the bug fixed wrongly back in 2014. It had potential to cause
> regression. We left it behind.
> * [8] Keep a list of soft-dirty part of a VMA across splits and merges. I
> got the reply don't increase the size of the VMA by 8 bytes.
> 
> At this point, we left soft-dirty considering it is too much delicate and
> userfaultfd [9] seemed like the only way forward. From there onward, we
> have been basing soft-dirty emulation on userfaultfd wp feature where
> kernel resolves the faults itself when WP_ASYNC feature is used. It was
> straight forward to add WP_ASYNC feature in userfautlfd. Now we get only
> those pages dirty or written-to which are really written in reality. (PS
> There is another WP_UNPOPULATED userfautfd feature is required which is
> needed to avoid pre-faulting memory before write-protecting [9].)
> 
> All the different masks were added on the request of CRIU devs to create
> interface more generic and better.
> 
> [1] https://learn.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-getwritewatch
> [2] https://lore.kernel.org/all/20221014134802.1361436-1-mdanylo@google.com
> [3] https://github.com/google/sanitizers
> [4] https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#64-bit
> [5] https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@redhat.com
> [6] https://lore.kernel.org/all/20220725142048.30450-1-peterx@redhat.com/
> [7] https://lore.kernel.org/all/20221122115007.2787017-1-usama.anjum@collabora.com
> [8] https://lore.kernel.org/all/20221220162606.1595355-1-usama.anjum@collabora.com
> [9] https://lore.kernel.org/all/20230306213925.617814-1-peterx@redhat.com
> [10] https://lore.kernel.org/all/20230125144529.1630917-1-mdanylo@google.com
> 
> * Original Cover letter from v8*
> Hello,
> 
> Note:
> Soft-dirty pages and pages which have been written-to are synonyms. As
> kernel already has soft-dirty feature inside which we have given up to
> use, we are using written-to terminology while using UFFD async WP under
> the hood.
> 
> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
> the info about page table entries. The following operations are
> supported in this ioctl:
> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>   (PAGE_IS_SWAPPED).
> - Write-protect the pages (PAGEMAP_WP_ENGAGE) to start finding which
>   pages have been written-to.
> - Find pages which have been written-to and write protect the pages
>   (atomic PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE)
> 
> It is possible to find and clear soft-dirty pages entirely in userspace.
> But it isn't efficient:
> - The mprotect and SIGSEGV handler for bookkeeping
> - The userfaultfd wp (synchronous) with the handler for bookkeeping
> 
> Some benchmarks can be seen here[1]. This series adds features that weren't
> present earlier:
> - There is no atomic get soft-dirty/Written-to status and clear present in
>   the kernel.
> - The pages which have been written-to can not be found in accurate way.
>   (Kernel's soft-dirty PTE bit + sof_dirty VMA bit shows more soft-dirty
>   pages than there actually are.)
> 
> Historically, soft-dirty PTE bit tracking has been used in the CRIU
> project. The procfs interface is enough for finding the soft-dirty bit
> status and clearing the soft-dirty bit of all the pages of a process.
> We have the use case where we need to track the soft-dirty PTE bit for
> only specific pages on-demand. We need this tracking and clear mechanism
> of a region of memory while the process is running to emulate the
> getWriteWatch() syscall of Windows.
> 
> *(Moved to using UFFD instead of soft-dirtyi feature to find pages which
> have been written-to from v7 patch series)*:
> Stop using the soft-dirty flags for finding which pages have been
> written to. It is too delicate and wrong as it shows more soft-dirty
> pages than the actual soft-dirty pages. There is no interest in
> correcting it [2][3] as this is how the feature was written years ago.
> It shouldn't be updated to changed behaviour. Peter Xu has suggested
> using the async version of the UFFD WP [4] as it is based inherently
> on the PTEs.
> 
> So in this patch series, I've added a new mode to the UFFD which is
> asynchronous version of the write protect. When this variant of the
> UFFD WP is used, the page faults are resolved automatically by the
> kernel. The pages which have been written-to can be found by reading
> pagemap file (!PM_UFFD_WP). This feature can be used successfully to
> find which pages have been written to from the time the pages were
> write protected. This works just like the soft-dirty flag without
> showing any extra pages which aren't soft-dirty in reality.
> 
> The information related to pages if the page is file mapped, present and
> swapped is required for the CRIU project [5][6]. The addition of the
> required mask, any mask, excluded mask and return masks are also required
> for the CRIU project [5].
> 
> The IOCTL returns the addresses of the pages which match the specific
> masks. The page addresses are returned in struct page_region in a compact
> form. The max_pages is needed to support a use case where user only wants
> to get a specific number of pages. So there is no need to find all the
> pages of interest in the range when max_pages is specified. The IOCTL
> returns when the maximum number of the pages are found. The max_pages is
> optional. If max_pages is specified, it must be equal or greater than the
> vec_size. This restriction is needed to handle worse case when one
> page_region only contains info of one page and it cannot be compacted.
> This is needed to emulate the Windows getWriteWatch() syscall.
> 
> The patch series include the detailed selftest which can be used as an
> example for the uffd async wp test and PAGEMAP_IOCTL. It shows the
> interface usages as well.
> 
> [1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@collabora.com/
> [2] https://lore.kernel.org/all/20221220162606.1595355-1-usama.anjum@collabora.com
> [3] https://lore.kernel.org/all/20221122115007.2787017-1-usama.anjum@collabora.com
> [4] https://lore.kernel.org/all/Y6Hc2d+7eTKs7AiH@x1n
> [5] https://lore.kernel.org/all/YyiDg79flhWoMDZB@gmail.com/
> [6] https://lore.kernel.org/all/20221014134802.1361436-1-mdanylo@google.com/
> 
> Regards,
> Muhammad Usama Anjum
> 
> Muhammad Usama Anjum (4):
>   fs/proc/task_mmu: Implement IOCTL to get and optionally clear info
>     about PTEs
>   tools headers UAPI: Update linux/fs.h with the kernel sources
>   mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL
>   selftests: mm: add pagemap ioctl tests
> 
> Peter Xu (1):
>   userfaultfd: UFFD_FEATURE_WP_ASYNC
> 
>  Documentation/admin-guide/mm/pagemap.rst     |   58 +
>  Documentation/admin-guide/mm/userfaultfd.rst |   35 +
>  fs/proc/task_mmu.c                           |  503 ++++++
>  fs/userfaultfd.c                             |   26 +-
>  include/linux/userfaultfd_k.h                |   21 +-
>  include/uapi/linux/fs.h                      |   53 +
>  include/uapi/linux/userfaultfd.h             |    9 +-
>  mm/hugetlb.c                                 |   32 +-
>  mm/memory.c                                  |   27 +-
>  tools/include/uapi/linux/fs.h                |   53 +
>  tools/testing/selftests/mm/.gitignore        |    1 +
>  tools/testing/selftests/mm/Makefile          |    3 +-
>  tools/testing/selftests/mm/config            |    1 +
>  tools/testing/selftests/mm/pagemap_ioctl.c   | 1459 ++++++++++++++++++
>  tools/testing/selftests/mm/run_vmtests.sh    |    4 +
>  15 files changed, 2262 insertions(+), 23 deletions(-)
>  create mode 100644 tools/testing/selftests/mm/pagemap_ioctl.c
>  mode change 100644 => 100755 tools/testing/selftests/mm/run_vmtests.sh
> 

-- 
BR,
Muhammad Usama Anjum

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