linux-kernel - [PATCH v11 3/7] userfaultfd: update documentation to describe UFFD_FEATURE_WP

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Date:   Thu,  9 Mar 2023 18:57:14 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: [PATCH v11 3/7] userfaultfd: update documentation to describe UFFD_FEATURE_WP_ASYNC

Explain the difference created by UFFD_FEATURE_WP_ASYNC to the write
protection (UFFDIO_WRITEPROTECT_MODE_WP) mode.

Suggested-by: Suggested-by: Peter Xu <peterx@...hat.com>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
---
Changes in v11:
- Update the documentation from reviews entirely from Peter
---
 Documentation/admin-guide/mm/userfaultfd.rst | 21 ++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 7dc823b56ca4..404d8aa8f09f 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -219,6 +219,27 @@ former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
 you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was
 used.
 
+If the userfaultfd context (that has ``UFFDIO_REGISTER_MODE_WP`` registered
+against) has ``UFFD_FEATURE_WP_ASYNC`` feature enabled, it will work in
+async write protection mode.  It can be seen as a more accurate version of
+soft-dirty tracking, meanwhile the results will not be easily affected by
+other operations like vma merging.
+
+Comparing to the generic mode, the async mode will not generate any
+userfaultfd message when the protected memory range is written.  Instead, the
+kernel will automatically resolve the page fault immediately by dropping the
+uffd-wp bit in the pgtables.  The user app can collect the "written/dirty"
+status by looking up the uffd-wp bit for the pages being interested in
+/proc/pagemap.
+
+The page will be under track of uffd-wp async mode until the page is explicitly
+write-protected by ``UFFDIO_WRITEPROTECT`` ioctl with the mode flag
+``UFFDIO_WRITEPROTECT_MODE_WP`` set.  Trying to resolve a page fault that was
+tracked by async mode userfaultfd-wp is invalid.
+
+Currently ``UFFD_FEATURE_WP_ASYNC`` only support anonymous and shmem. Hugetlb is
+not yet supported.
+
 QEMU/KVM
 ========
 
-- 
2.39.2