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

[Andrei Vagin <avagin@gmail.com>]

On Fri, Aug 26, 2022 at 11:45:31AM +0500, 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.

I think this interface doesn't have to be limited by the soft-dirty
bits only. For example, CRIU needs to know whether file, present and swap bits
are set or not.

I mean we should be able to specify for what pages we need to get info
for. An ioctl argument can have these four fields:
* required bits (rmask & mask == mask) - all bits from this mask have to be set.
* any of these bits (amask & mask != 0) - any of these bits is set.
* exclude masks (emask & mask == 0) = none of these bits are set.
* return mask - bits that have to be reported to user.

> - Clear the pages which are soft-dirty.
> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
> soft-dirty PTE bit
> 
> 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

We can conside to return regions that contains pages with the same set
of bits.

struct page_region {
	void *start;
	long size;
	u64 bitmap;
}

And ioctl returns arrays of page_region-s. I believe it will be more
compact form for many cases.

> - 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
> 
> -- 
> 2.30.2
>