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

Richard Weinberger
2024-03-06T23:24:07Z

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

Signed-off-by: Richard Weinberger <richard@nod.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