From patchwork Tue Jul 11 12:52:40 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Muhammad Usama Anjum X-Patchwork-Id: 13308664 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 D95FAC001DD for ; Tue, 11 Jul 2023 12:54:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231345AbjGKMyH (ORCPT ); Tue, 11 Jul 2023 08:54:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54920 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232329AbjGKMxu (ORCPT ); Tue, 11 Jul 2023 08:53:50 -0400 Received: from madras.collabora.co.uk (madras.collabora.co.uk [IPv6:2a00:1098:0:82:1000:25:2eeb:e5ab]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 53078E49; Tue, 11 Jul 2023 05:53:48 -0700 (PDT) Received: from localhost.localdomain (unknown [119.155.40.122]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: usama.anjum) by madras.collabora.co.uk (Postfix) with ESMTPSA id B61B5660217D; Tue, 11 Jul 2023 13:53:39 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1689080027; bh=e3RwcWLS0DN4jGAQ1nXg3KEjRQ1a6mnaZglmii6vuAs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MdARlwDFQDKScisXGCtEwJ9rzc9ku5dA1jLj2OZ1T00ExwO5tt2SmfX1hEFuAsd9+ 6LeHKU7sHQoFP2XdfPRRF6vEThylU/M7M/rM9KfOsOqm8k5/fgryA8ToQZ2t9iiJge cq3UOpVkRrhLNthVwSvFMjCp2MlXgw7HH+aj6Loi8jrF4WkHUqFK2Un4CafgcVW7sH 4A+iRmTMYrZpp5i/iKd+Gi5wyO6V04TBMnvbK579rqxsQJEnsn7ZzaQee8t0K7eA6y jPDvLFjgWAvV6USfK7nDQqjt3QHIky7ng7QUfXfC47leRaLQQTb4aNMXAyRfyuggIu JnpJFXyQF1ldw== From: Muhammad Usama Anjum To: Peter Xu , David Hildenbrand , Andrew Morton , =?utf-8?b?TWljaGHFgiBNaXJvc8WC?= =?utf-8?b?YXc=?= , 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 , Muhammad Usama Anjum , 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 Subject: [PATCH v24 4/5] mm/pagemap: add documentation of PAGEMAP_SCAN IOCTL Date: Tue, 11 Jul 2023 17:52:40 +0500 Message-Id: <20230711125241.1587820-5-usama.anjum@collabora.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230711125241.1587820-1-usama.anjum@collabora.com> References: <20230711125241.1587820-1-usama.anjum@collabora.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Add some explanation and method to use write-protection and written-to on memory range. Signed-off-by: Muhammad Usama Anjum --- 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 rst --- 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..4bdad8eff009 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``), swapped + (``PAGE_IS_SWAPPED``) or page has pfn zero (``PAGE_IS_PFNZERO``). +- 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 ``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 ``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`` unset, ``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 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.