From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8F1EEB64DB for ; Wed, 14 Jun 2023 04:20:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242103AbjFNEUc (ORCPT ); Wed, 14 Jun 2023 00:20:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33408 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233845AbjFNEU3 (ORCPT ); Wed, 14 Jun 2023 00:20:29 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:3::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ACC2119B1; Tue, 13 Jun 2023 21:20:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Sender:Reply-To:Content-ID:Content-Description; bh=D9e3x9i8rBKYm3toYXDRWUXq9796QOZEiqel4E9RDSQ=; b=ptry9IrRJnz/Kqmd1+0osrL1CQ iSA86OAjUouPrH3MIaIdajDF8MeIKN16meKgjfkpQXUtlmG7+YfL6VwgPDRLPo2YUvfn4WZ4hGNI+ DzkxISScnE7H0Hm70ysbCXZkG6iQcxhJst84nH5UEwSVp4ZS/TyvG8/3/GnsTNa/Nu2+5EXlQruEz GAMeK7rfui/COGK3C8/0lpbQYHJB79vqlU7aK02zJhMrzjKLaMKyWXXTnhRilEDJMxPfkuiWHaDVX ED9HhmateLa2wPxWCmaSd08C2s/sflld2Y2YwrlC4Ss6MkQRCL/5T8TIGihDngwMXn5db+ottR/U6 aDuqAwpw==; Received: from [2601:1c2:980:9ec0::2764] by bombadil.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1q9HzO-009xON-2S; Wed, 14 Jun 2023 04:20:10 +0000 Message-ID: <57358041-0e36-deaa-e2ec-06dcfc92ba4d@infradead.org> Date: Tue, 13 Jun 2023 21:20:09 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.11.2 Subject: Re: [PATCH v18 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Content-Language: en-US To: Muhammad Usama Anjum , Peter Xu , David Hildenbrand , Andrew Morton , =?UTF-8?B?TWljaGHFgiBNaXJvc8WC?= =?UTF-8?Q?aw?= , Andrei Vagin , Danylo Mocherniuk , Paul Gofman , Cyrill Gorcunov , Mike Rapoport , Nadav Amit Cc: Alexander Viro , Shuah Khan , Christian Brauner , Yang Shi , Vlastimil Babka , "Liam R . Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com References: <20230613102905.2808371-1-usama.anjum@collabora.com> <20230613102905.2808371-5-usama.anjum@collabora.com> From: Randy Dunlap In-Reply-To: <20230613102905.2808371-5-usama.anjum@collabora.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > --- > 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