| Message ID | 20230809061603.1969154-6-usama.anjum@collabora.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Implement IOCTL to get and optionally clear info about PTEs | expand |
On Wed, 9 Aug 2023 at 08:16, Muhammad Usama Anjum <usama.anjum@collabora.com> wrote: > Add some explanation and method to use write-protection and written-to > on memory range. [...] > --- a/Documentation/admin-guide/mm/pagemap.rst > +++ b/Documentation/admin-guide/mm/pagemap.rst > @@ -227,3 +227,67 @@ 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 Async Write-Protection enabled > + (``PAGE_IS_WPALLOWED``), have been written to (``PAGE_IS_WRITTEN``), file mapped > + (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped (``PAGE_IS_SWAPPED``) > + or page has pfn zero (``PAGE_IS_PFNZERO``). A recent addition -- PAGE_IS_HUGE -- is missing. BTW, it could be easier to understand if the page categories were separated from the operation description and listed so that each has its own line and maybe a longer description where needed. > +- Find pages which have been written to and/or write protect > + (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages atomically. > + The (``PM_SCAN_WP_MATCHING``) is used to WP the matched pages. The > + (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if non-Async-Write-Protected > + pages are found. The operation the IOCTL does now is: "scan the process page tables and report memory ranges matching provided criteria '. Flags extend the operation: "PM_SCAN_WP_MATCHING write protects the memory reported" (it does it atomically, but this is just an optimization, isn't it? A process could gather the ranges, WP them, and then copy.) "PM_SCAN_CHECK_WPASYNC" aborts the scan early if a non-WP-able matching page is found. > +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_WP_MATCHING`` > + and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get > + operation is optionally performed depending upon if the output buffer is > + provided or not. > + 3. The range is specified through ``start`` and ``end``. > + 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 ``category_mask``, ``category_anyof_mask``, > + ``category_inverted`` 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`` unset, ``category_mask`` > + is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``, ``category_inverted`` 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, ``category_anyof_mask`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``. > + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is > + specified in ``category_mask`` and ``return_mask``. In addition to > + specifying the output buffer in ``vec`` and length in ``vec_len``, the > + ``PM_SCAN_WP_MATCHING`` is specified in ``flags`` to perform write protect > + on the range as well. Could this be rewritten as examples? E.g.: Finding dirty file-backed pages: struct pm_scan_arg arg = { .size = sizeof(arg), .flags = 0, ... .category_mask = ..., .return_mask = ... }; ssize_t n = ioctl(..., &arg); Find dirty pages and write protect them in the same call: arg = { ... }; do { ... ioctl(...) } while(...); (The code snippets heavily commented.) > +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct "as a better-performing alternative" > +alternative of soft-dirty flag. It doesn't get affected by housekeeping chores > +(VMA merging) of the kernel and hence the user can find the true soft-dirty pages > +only. This is still an optimization, e.g. in THP case there might be too many pages reported? > + 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. This repeats the description of PM_SCAN_WP_MATCHING -- I suggest removing this part. > +To get information about which pages have been written to or optionally write > +protect the pages, following must be performed first in order: "PAGE_IS_WRITTEN" category is used with uffd write protect-enabled ranges to implement memory dirty tracking in userspace: > + 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 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. I guess that the UFFD performance could be fixed? But this part refers to the old PM_SCAN_OP_WP, so an updated example is needed. > + 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. "find the pages written to since they were last write protected", but this sounds contradicting: we look for pages that were WP but written anyway. (IOW: marking write-protected is an implementation detail - the ioctl is to find pages that changed since they were last marked.) Maybe we should call the operation "marking CLEAN" or alike? Best Regards Michał Mirosław
On 8/11/23 12:26 AM, Michał Mirosław wrote: > On Wed, 9 Aug 2023 at 08:16, Muhammad Usama Anjum > <usama.anjum@collabora.com> wrote: >> Add some explanation and method to use write-protection and written-to >> on memory range. > [...] >> --- a/Documentation/admin-guide/mm/pagemap.rst >> +++ b/Documentation/admin-guide/mm/pagemap.rst >> @@ -227,3 +227,67 @@ 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 Async Write-Protection enabled >> + (``PAGE_IS_WPALLOWED``), have been written to (``PAGE_IS_WRITTEN``), file mapped >> + (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped (``PAGE_IS_SWAPPED``) >> + or page has pfn zero (``PAGE_IS_PFNZERO``). > > A recent addition -- PAGE_IS_HUGE -- is missing. > > BTW, it could be easier to understand if the page categories were > separated from the operation description and listed so that each has > its own line and maybe a longer description where needed. I've made 90% of changes you have asked in documentation. > >> +- Find pages which have been written to and/or write protect >> + (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages atomically. >> + The (``PM_SCAN_WP_MATCHING``) is used to WP the matched pages. The >> + (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if non-Async-Write-Protected >> + pages are found. > > The operation the IOCTL does now is: "scan the process page tables and > report memory ranges matching provided criteria '. > Flags extend the operation: "PM_SCAN_WP_MATCHING write protects the > memory reported" (it does it atomically, but this is just an > optimization, isn't it? A process could gather the ranges, WP them, > and then copy.) > "PM_SCAN_CHECK_WPASYNC" aborts the scan early if a non-WP-able > matching page is found. > >> +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_WP_MATCHING`` >> + and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get >> + operation is optionally performed depending upon if the output buffer is >> + provided or not. >> + 3. The range is specified through ``start`` and ``end``. >> + 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 ``category_mask``, ``category_anyof_mask``, >> + ``category_inverted`` 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`` unset, ``category_mask`` >> + is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``, ``category_inverted`` 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, ``category_anyof_mask`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``. >> + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is >> + specified in ``category_mask`` and ``return_mask``. In addition to >> + specifying the output buffer in ``vec`` and length in ``vec_len``, the >> + ``PM_SCAN_WP_MATCHING`` is specified in ``flags`` to perform write protect >> + on the range as well. > > Could this be rewritten as examples? E.g.: > > Finding dirty file-backed pages: > > struct pm_scan_arg arg = { > .size = sizeof(arg), > .flags = 0, > ... > .category_mask = ..., > .return_mask = ... > }; > ssize_t n = ioctl(..., &arg); > > Find dirty pages and write protect them in the same call: > > arg = { ... }; > do { > ... ioctl(...) > } while(...); > > (The code snippets heavily commented.) > >> +The ``PAGE_IS_WRITTEN`` flag can be considered as the better and correct > > "as a better-performing alternative" > >> +alternative of soft-dirty flag. It doesn't get affected by housekeeping chores >> +(VMA merging) of the kernel and hence the user can find the true soft-dirty pages >> +only. > > This is still an optimization, e.g. in THP case there might be too > many pages reported? > >> + 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. > > This repeats the description of PM_SCAN_WP_MATCHING -- I suggest > removing this part. > >> +To get information about which pages have been written to or optionally write >> +protect the pages, following must be performed first in order: > > "PAGE_IS_WRITTEN" category is used with uffd write protect-enabled > ranges to implement memory dirty tracking in userspace: > >> + 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 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. > > I guess that the UFFD performance could be fixed? But this part refers > to the old PM_SCAN_OP_WP, so an updated example is needed. > >> + 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. > > "find the pages written to since they were last write protected", but > this sounds contradicting: we look for pages that were WP but written > anyway. (IOW: marking write-protected is an implementation detail - > the ioctl is to find pages that changed since they were last marked.) > Maybe we should call the operation "marking CLEAN" or alike? > > Best Regards > Michał Mirosław
diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst index c8f380271cad8..ea18e4575902a 100644 --- a/Documentation/admin-guide/mm/pagemap.rst +++ b/Documentation/admin-guide/mm/pagemap.rst @@ -227,3 +227,67 @@ 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 Async Write-Protection enabled + (``PAGE_IS_WPALLOWED``), have been written to (``PAGE_IS_WRITTEN``), file mapped + (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped (``PAGE_IS_SWAPPED``) + or page has pfn zero (``PAGE_IS_PFNZERO``). +- Find pages which have been written to and/or write protect + (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages atomically. + The (``PM_SCAN_WP_MATCHING``) is used to WP the matched pages. The + (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if non-Async-Write-Protected + pages are found. + +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_WP_MATCHING`` + and ``PM_SCAN_CHECK_WPASYNC`` are the only added flags at this time. The get + operation is optionally performed depending upon if the output buffer is + provided or not. + 3. The range is specified through ``start`` and ``end``. + 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 ``category_mask``, ``category_anyof_mask``, + ``category_inverted`` 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`` unset, ``category_mask`` + is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``, ``category_inverted`` 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, ``category_anyof_mask`` is set to ``PAGE_IS_FILE | PAGE_IS_SWAPPED``. + 3. To find written pages and engage write protect, ``PAGE_IS_WRITTEN`` is + specified in ``category_mask`` and ``return_mask``. In addition to + specifying the output buffer in ``vec`` and length in ``vec_len``, the + ``PM_SCAN_WP_MATCHING`` 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 housekeeping 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 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.
Add some explanation and method to use write-protection and written-to on memory range. Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com> --- Changes in v26: - Update documentation according to latest revision Changes in v21: - Update according to changed interface Changes in v20: - Add new flag in documentation Changes in v19: - Improved documentatiom with punctuation marks Changes in v16: - Update the documentation Changes in v11: - Add more documentation --- Documentation/admin-guide/mm/pagemap.rst | 64 ++++++++++++++++++++++++ 1 file changed, 64 insertions(+)