On 2/10/23 12:26 AM, Peter Xu wrote: > On Thu, Feb 02, 2023 at 04:29:14PM +0500, 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 <usama.anjum@xxxxxxxxxxxxx> >> --- >> Documentation/admin-guide/mm/pagemap.rst | 24 ++++++++++++++++++++++++ >> 1 file changed, 24 insertions(+) >> >> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst >> index 6e2e416af783..1cb2189e9a0d 100644 >> --- a/Documentation/admin-guide/mm/pagemap.rst >> +++ b/Documentation/admin-guide/mm/pagemap.rst >> @@ -230,3 +230,27 @@ 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 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``) > > Could we extend this section a bit more? Some points for reference: > > - The new struct you introduced, definitions of each of the fields, and > generic use cases for each of the field/ops. > > - It'll be nice to list the OPs the new interface supports (GET, > WP_ENGAGE, GET+WP_ENGAGE). > > - When should people use this rather than the old pagemap interface? > What's the major problems to solve / what's the major difference? > (Maybe nice to reference the Windows API too here) I'll update the documentation. > >> + >> +To get information about which pages have been written-to and/or 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_ASYNC`` feature is set by ``UFFDIO_API`` IOCTL. >> + 3. The memory range is registered with ``UFFDIO_REGISTER_MODE_WP`` mode >> + through ``UFFDIO_REGISTER`` IOCTL. >> +Then the any part of the registered memory or the whole memory region can be >> +write protected using the ``UFFDIO_WRITEPROTECT`` IOCTL or ``PAGEMAP_SCAN`` >> +IOCTL. > > This part looks good. > > Thanks, > -- BR, Muhammad Usama Anjum
