Add some explanation and method to use write-protection and written-to
on memory range.
Signed-off-by: Muhammad Usama Anjum <[email protected]>
---
Changes in v16:
- Update the documentation
Changes in v11:
- Add more documentation
---
Documentation/admin-guide/mm/pagemap.rst | 58 ++++++++++++++++++++++++
1 file changed, 58 insertions(+)
diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index c8f380271cad..3b977526d4b3 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -227,3 +227,61 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
always 12 at most architectures). Since Linux 3.11 their meaning changes
after first clear of soft-dirty bits. Since Linux 4.2 they are used for
flags unconditionally.
+
+Pagemap Scan IOCTL
+==================
+
+The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally
+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``).
+- Find pages which have been written-to and/or write protect the pages atomically
+ (atomic ``PM_SCAN_OP_GET + PM_SCAN_OP_WP``)
+
+The ``struct pm_scan_arg`` is used as the argument of the IOCTL.
+ 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
+ field. This field will be helpful in recognizing the structure if extensions
+ are done later.
+ 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_OP_GET``
+ and ``PM_SCAN_OP_WP`` are the only added flags at this time.
+ 3. The range is specified through ``start`` and ``len``.
+ 4. The output buffer of ``struct page_region`` array and size is specified in
+ ``vec`` and ``vec_len``.
+ 5. The optional maximum requested pages are specified in the ``max_pages``.
+ 6. The masks are specified in ``required_mask``, ``anyof_mask``,
+ ``excluded_ mask`` and ``return_mask``.
+ 1. To find if ``PAGE_IS_WRITTEN`` flag is set for pages which have
+ ``PAGE_IS_FILE`` set and ``PAGE_IS_SWAPPED`` un-set, ``required_mask``
+ is set to ``PAGE_IS_FILE``, ``exclude_mask`` is set to
+ ``PAGE_IS_SWAPPED`` and ``return_mask`` is set to ``PAGE_IS_WRITTEN``.
+ The output buffer in ``vec`` and length must be specified in ``vec_len``.
+ 2. To find pages which have either ``PAGE_IS_FILE`` or ``PAGE_IS_SWAPPED``
+ set, ``anyof_masks`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``.
+ 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is
+ specified in ``required_mask`` and ``return_mask``. In addition to
+ specifying the output buffer in ``vec`` and length in ``vec_len``, the
+ ``PM_SCAN_OP_WP`` is specified in ``flags`` to perform write protect
+ on the range as well.
+
+The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct
+alternative of soft-dirty flag. It doesn't get affected by household chores (VMA
+merging) of the kernel and hence the user can find the true soft-dirty pages
+only. This IOCTL adds the atomic way to find which pages have been written and
+write protect those pages again. This kind of operation is needed to efficiently
+find out which pages have changed in the memory.
+
+To get information about which pages have been written-to or optionally write
+protect the pages, following must be performed first in order:
+ 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
+ 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
+ are set by ``UFFDIO_API`` IOCTL.
+ 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
+ through ``UFFDIO_REGISTER`` IOCTL.
+ 4. Then the any part of the registered memory or the whole memory region must
+ be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_OP_WP``
+ or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
+ same operation. The former is better in terms of performance.
+ 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
+ have been written-to and/or optionally write protect the pages as well.
--
2.39.2
Hi--
On 6/13/23 03:29, Muhammad Usama Anjum wrote:
> Add some explanation and method to use write-protection and written-to
> on memory range.
>
> Signed-off-by: Muhammad Usama Anjum <[email protected]>
> ---
> Changes in v16:
> - Update the documentation
>
> Changes in v11:
> - Add more documentation
> ---
> Documentation/admin-guide/mm/pagemap.rst | 58 ++++++++++++++++++++++++
> 1 file changed, 58 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index c8f380271cad..3b977526d4b3 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -227,3 +227,61 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
> always 12 at most architectures). Since Linux 3.11 their meaning changes
> after first clear of soft-dirty bits. Since Linux 4.2 they are used for
> flags unconditionally.
> +
> +Pagemap Scan IOCTL
> +==================
> +
> +The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally
> +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``),
written to
> + file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``) or swapped
> + (``PAGE_IS_SWAPPED``).
> +- Find pages which have been written-to and/or write protect the pages atomically
written to
> + (atomic ``PM_SCAN_OP_GET + PM_SCAN_OP_WP``)
> +
> +The ``struct pm_scan_arg`` is used as the argument of the IOCTL.
> + 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
> + field. This field will be helpful in recognizing the structure if extensions
> + are done later.
> + 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_OP_GET``
> + and ``PM_SCAN_OP_WP`` are the only added flags at this time.
> + 3. The range is specified through ``start`` and ``len``.
> + 4. The output buffer of ``struct page_region`` array and size is specified in
> + ``vec`` and ``vec_len``.
> + 5. The optional maximum requested pages are specified in the ``max_pages``.
> + 6. The masks are specified in ``required_mask``, ``anyof_mask``,
> + ``excluded_ mask`` and ``return_mask``.
> + 1. To find if ``PAGE_IS_WRITTEN`` flag is set for pages which have
> + ``PAGE_IS_FILE`` set and ``PAGE_IS_SWAPPED`` un-set, ``required_mask``
unset,
> + is set to ``PAGE_IS_FILE``, ``exclude_mask`` is set to
> + ``PAGE_IS_SWAPPED`` and ``return_mask`` is set to ``PAGE_IS_WRITTEN``.
> + The output buffer in ``vec`` and length must be specified in ``vec_len``.
> + 2. To find pages which have either ``PAGE_IS_FILE`` or ``PAGE_IS_SWAPPED``
> + set, ``anyof_masks`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``.
> + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is
> + specified in ``required_mask`` and ``return_mask``. In addition to
> + specifying the output buffer in ``vec`` and length in ``vec_len``, the
> + ``PM_SCAN_OP_WP`` is specified in ``flags`` to perform write protect
> + on the range as well.
> +
> +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct
> +alternative of soft-dirty flag. It doesn't get affected by household chores (VMA
housekeeping ?
> +merging) of the kernel and hence the user can find the true soft-dirty pages
> +only. This IOCTL adds the atomic way to find which pages have been written and
> +write protect those pages again. This kind of operation is needed to efficiently
> +find out which pages have changed in the memory.
> +
> +To get information about which pages have been written-to or optionally write
written to
> +protect the pages, following must be performed first in order:
> + 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
> + 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
> + are set by ``UFFDIO_API`` IOCTL.
> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
> + through ``UFFDIO_REGISTER`` IOCTL.
> + 4. Then the any part of the registered memory or the whole memory region must
"the" or "any". Probably "any". Not both.
> + be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_OP_WP``
> + or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
> + same operation. The former is better in terms of performance.
> + 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
> + have been written-to and/or optionally write protect the pages as well.
written to
--
~Randy
Hi Randy,
Thank you for your review.
On 6/14/23 9:20 AM, Randy Dunlap wrote:
> Hi--
>
> On 6/13/23 03:29, Muhammad Usama Anjum wrote:
>> Add some explanation and method to use write-protection and written-to
>> on memory range.
>>
>> Signed-off-by: Muhammad Usama Anjum <[email protected]>
>> ---
>> Changes in v16:
>> - Update the documentation
>>
>> Changes in v11:
>> - Add more documentation
>> ---
>> Documentation/admin-guide/mm/pagemap.rst | 58 ++++++++++++++++++++++++
>> 1 file changed, 58 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
>> index c8f380271cad..3b977526d4b3 100644
>> --- a/Documentation/admin-guide/mm/pagemap.rst
>> +++ b/Documentation/admin-guide/mm/pagemap.rst
>> @@ -227,3 +227,61 @@ Before Linux 3.11 pagemap bits 55-60 were used for "page-shift" (which is
>> always 12 at most architectures). Since Linux 3.11 their meaning changes
>> after first clear of soft-dirty bits. Since Linux 4.2 they are used for
>> flags unconditionally.
>> +
>> +Pagemap Scan IOCTL
>> +==================
>> +
>> +The ``PAGEMAP_SCAN`` IOCTL on the pagemap file can be used to get or optionally
>> +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``),
>
> written to
I'll make updates on all the places you have suggested.
>
>> + file mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``) or swapped
>> + (``PAGE_IS_SWAPPED``).
>> +- Find pages which have been written-to and/or write protect the pages atomically
>
> written to
>
>> + (atomic ``PM_SCAN_OP_GET + PM_SCAN_OP_WP``)
>> +
>> +The ``struct pm_scan_arg`` is used as the argument of the IOCTL.
>> + 1. The size of the ``struct pm_scan_arg`` must be specified in the ``size``
>> + field. This field will be helpful in recognizing the structure if extensions
>> + are done later.
>> + 2. The flags can be specified in the ``flags`` field. The ``PM_SCAN_OP_GET``
>> + and ``PM_SCAN_OP_WP`` are the only added flags at this time.
>> + 3. The range is specified through ``start`` and ``len``.
>> + 4. The output buffer of ``struct page_region`` array and size is specified in
>> + ``vec`` and ``vec_len``.
>> + 5. The optional maximum requested pages are specified in the ``max_pages``.
>> + 6. The masks are specified in ``required_mask``, ``anyof_mask``,
>> + ``excluded_ mask`` and ``return_mask``.
>> + 1. To find if ``PAGE_IS_WRITTEN`` flag is set for pages which have
>> + ``PAGE_IS_FILE`` set and ``PAGE_IS_SWAPPED`` un-set, ``required_mask``
>
> unset,
>
>> + is set to ``PAGE_IS_FILE``, ``exclude_mask`` is set to
>> + ``PAGE_IS_SWAPPED`` and ``return_mask`` is set to ``PAGE_IS_WRITTEN``.
>> + The output buffer in ``vec`` and length must be specified in ``vec_len``.
>> + 2. To find pages which have either ``PAGE_IS_FILE`` or ``PAGE_IS_SWAPPED``
>> + set, ``anyof_masks`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``.
>> + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is
>> + specified in ``required_mask`` and ``return_mask``. In addition to
>> + specifying the output buffer in ``vec`` and length in ``vec_len``, the
>> + ``PM_SCAN_OP_WP`` is specified in ``flags`` to perform write protect
>> + on the range as well.
>> +
>> +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct
>> +alternative of soft-dirty flag. It doesn't get affected by household chores (VMA
>
> housekeeping ?
>
>> +merging) of the kernel and hence the user can find the true soft-dirty pages
>> +only. This IOCTL adds the atomic way to find which pages have been written and
>> +write protect those pages again. This kind of operation is needed to efficiently
>> +find out which pages have changed in the memory.
>> +
>> +To get information about which pages have been written-to or optionally write
>
> written to
>
>> +protect the pages, following must be performed first in order:
>> + 1. The userfaultfd file descriptor is created with ``userfaultfd`` syscall.
>> + 2. The ``UFFD_FEATURE_WP_UNPOPULATED`` and ``UFFD_FEATURE_WP_ASYNC`` features
>> + are set by ``UFFDIO_API`` IOCTL.
>> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode
>> + through ``UFFDIO_REGISTER`` IOCTL.
>> + 4. Then the any part of the registered memory or the whole memory region must
>
> "the" or "any". Probably "any". Not both.
>
>> + be write protected using ``PAGEMAP_SCAN`` IOCTL with flag ``PM_SCAN_OP_WP``
>> + or the ``UFFDIO_WRITEPROTECT`` IOCTL can be used. Both of these perform the
>> + same operation. The former is better in terms of performance.
>> + 5. Now the ``PAGEMAP_SCAN`` IOCTL can be used to either just find pages which
>> + have been written-to and/or optionally write protect the pages as well.
>
> written to
>
>
--
BR,
Muhammad Usama Anjum