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

David Hildenbrand
2024-03-07T10:52:51Z

On 07.03.24 00:23, Richard Weinberger wrote:
> 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.

That is not true.

--
Cheers,

David / dhildenb