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

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Message-ID: <db29666a-32b8-4bf6-ab13-7de3b09b0da1@redhat.com>
Date: Thu, 7 Mar 2024 11:52:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Richard Weinberger <richard@....at>, 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, avagin@...gle.com, lstoakes@...il.com,
 vbabka@...e.cz, akpm@...ux-foundation.org, usama.anjum@...labora.com,
 corbet@....net
Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit

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@....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