linux-kernel - Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

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Message-ID: <19df8db9-cff4-848e-905b-525f620848e8@collabora.com>
Date:   Wed, 7 Sep 2022 14:40:37 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:PROC FILESYSTEM" <linux-fsdevel@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Cc:     usama.anjum@...labora.com, kernel@...labora.com,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

On 8/26/22 11:45 AM, Muhammad Usama Anjum wrote:
> 
> Hello,
> 
> This patch series implements a new ioctl on the pagemap proc fs file to
> get, clear and perform both get and clear at the same time atomically on
> the specified range of the memory.
> 
> Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
> procfs file. The soft-dirty PTE bit for the whole memory range of the
> process can be cleared by writing to the clear_refs file. This series
> adds features that weren't present earlier.
> - There is no atomic get soft-dirty PTE bit status and clear operation
>   present.
> - The soft-dirty PTE bit of only a part of memory cannot be cleared.
> 
> Historically, soft-dirty PTE bit tracking has been used in the CRIU
> project. The proc fs interface is enough for that as I think the process
> is frozen. We have the use case where we need to track the soft-dirty
> PTE bit for the running processes. We need this tracking and clear
> mechanism of a region of memory while the process is running to emulate
> the getWriteWatch() syscall of Windows. This syscall is used by games to
> keep track of dirty pages and keep processing only the dirty pages. This
> new ioctl can be used by the CRIU project and other applications which
> require soft-dirty PTE bit information.
> 
> As in the current kernel there is no way to clear a part of memory (instead
> of clearing the Soft-Dirty bits for the entire process) and get+clear
> operation cannot be performed atomically, there are other methods to mimic
> this information entirely in userspace with poor performance:
> - The mprotect syscall and SIGSEGV handler for bookkeeping
> - The userfaultfd syscall with the handler for bookkeeping
> Some benchmarks can be seen [1].
> 
> This ioctl can be used by the CRIU project and other applications which
> require soft-dirty PTE bit information. The following operations are
> supported in this ioctl:
> - Get the pages that are soft-dirty.
> - Clear the pages which are soft-dirty.
> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
> soft-dirty PTE bit
Thoughts?

> 
> There are two decisions which have been taken about how to get the output
> from the syscall.
> - Return offsets of the pages from the start in the vec
> - Stop execution when vec is filled with dirty pages
> These two arguments doesn't follow the mincore() philosophy where the
> output array corresponds to the address range in one to one fashion, hence
> the output buffer length isn't passed and only a flag is set if the page
> is present. This makes mincore() easy to use with less control. We are
> passing the size of the output array and putting return data consecutively
> which is offset of dirty pages from the start. The user can convert these
> offsets back into the dirty page addresses easily. Suppose, the user want
> to get first 10 dirty pages from a total memory of 100 pages. He'll
> allocate output buffer of size 10 and the ioctl will abort after finding the
> 10 pages. This behaviour is needed to support Windows' getWriteWatch(). The
> behaviour like mincore() can be achieved by passing output buffer of 100
> size. This interface can be used for any desired behaviour.
> 
> [1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@collabora.com/
> 
> Regards,
> Muhammad Usama Anjum
> 
> Muhammad Usama Anjum (4):
>   fs/proc/task_mmu: update functions to clear the soft-dirty PTE bit
>   fs/proc/task_mmu: Implement IOCTL to get and clear soft dirty PTE bit
>   selftests: vm: add pagemap ioctl tests
>   mm: add documentation of the new ioctl on pagemap
> 
>  Documentation/admin-guide/mm/soft-dirty.rst |  42 +-
>  fs/proc/task_mmu.c                          | 342 ++++++++++-
>  include/uapi/linux/fs.h                     |  23 +
>  tools/include/uapi/linux/fs.h               |  23 +
>  tools/testing/selftests/vm/.gitignore       |   1 +
>  tools/testing/selftests/vm/Makefile         |   2 +
>  tools/testing/selftests/vm/pagemap_ioctl.c  | 649 ++++++++++++++++++++
>  7 files changed, 1050 insertions(+), 32 deletions(-)
>  create mode 100644 tools/testing/selftests/vm/pagemap_ioctl.c
> 

-- 
Muhammad Usama Anjum