linux-kernel - [PATCH v2 6/6] mm: userfaultfd: add basic documentation for UFFDIO

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Message-ID: <20230629205040.665834-6-axelrasmussen@google.com>
Date:   Thu, 29 Jun 2023 13:50:40 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Huang Ying <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        James Houghton <jthoughton@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Nadav Amit <namit@...are.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
        ZhangPeng <zhangpeng362@...wei.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: [PATCH v2 6/6] mm: userfaultfd: add basic documentation for UFFDIO_POISON

Just describe the feature at a really basic level.

Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 7c304e432205..b19053436369 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -244,6 +244,21 @@ write-protected (so future writes will also result in a WP fault). These ioctls
 support a mode flag (``UFFDIO_COPY_MODE_WP`` or ``UFFDIO_CONTINUE_MODE_WP``
 respectively) to configure the mapping this way.
 
+Memory Poisioning Emulation
+---------------------------
+
+In response to a fault (either missing or minor), an action userspace can
+take to "resolve" it is to issue a ``UFFDIO_POISON``. This will cause any
+future faulters to either get a SIGBUS, or in KVM's case the guest will
+receive an MCE as if there were hardware memory poisoning.
+
+This is used to emulate hardware memory poisoning. Imagine a VM running on a
+machine which experiences a real hardware memory error. Later, we live migrate
+the VM to another physical machine. Since we want the migration to be
+transparent to the guest, we want that same address range to act as if it was
+still poisoned, even though it's on a new physical host which ostentisbly
+doesn't have a memory error in the exact same spot.
+
 QEMU/KVM
 ========
 
-- 
2.41.0.255.g8b1d071c50-goog