linux-kernel - [PATCH 2/2] [RFC] pagemap.rst: Document write bit

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Date: Thu,  7 Mar 2024 00:23:39 +0100
From: Richard Weinberger <richard@....at>
To: linux-mm@...ck.org
Cc: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	upstream+pagemap@...ma-star.at,
	adobriyan@...il.com,
	wangkefeng.wang@...wei.com,
	ryan.roberts@....com,
	hughd@...gle.com,
	peterx@...hat.com,
	david@...hat.com,
	avagin@...gle.com,
	lstoakes@...il.com,
	vbabka@...e.cz,
	akpm@...ux-foundation.org,
	usama.anjum@...labora.com,
	corbet@....net,
	Richard Weinberger <richard@....at>
Subject: [PATCH 2/2] [RFC] pagemap.rst: Document write bit

Bit 58 denotes that a PTE is writable.
The main use case is detecting CoW mappings.

Signed-off-by: Richard Weinberger <richard@....at>
---
 Documentation/admin-guide/mm/pagemap.rst | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index f5f065c67615..81ffe3601b96 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -21,7 +21,8 @@ There are four components to pagemap:
     * Bit  56    page exclusively mapped (since 4.2)
     * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
       Documentation/admin-guide/mm/userfaultfd.rst)
-    * Bits 58-60 zero
+    * Bit  58    pte is writable (since 6.10)
+    * Bits 59-60 zero
     * Bit  61    page is file-page or shared-anon (since 3.5)
     * Bit  62    page swapped
     * Bit  63    page present
@@ -37,6 +38,11 @@ There are four components to pagemap:
    precisely which pages are mapped (or in swap) and comparing mapped
    pages between processes.
 
+   Bit 58 is useful to detect CoW mappings; however, it does not indicate
+   whether the page mapping is writable or not. If an anonymous mapping is
+   writable but the write bit is not set, it means that the next write access
+   will cause a page fault, and copy-on-write will happen.
+
    Efficient users of this interface will use ``/proc/pid/maps`` to
    determine which areas of memory are actually mapped and llseek to
    skip over unmapped regions.
-- 
2.35.3