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[2003:cb:c74d:6400:4867:4ed0:9726:a0c9]) by smtp.gmail.com with ESMTPSA id r12-20020adff10c000000b0033de2f2a88dsm20096450wro.103.2024.03.07.02.52.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Mar 2024 02:52:47 -0800 (PST) Message-ID: Date: Thu, 7 Mar 2024 11:52:46 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit Content-Language: en-US To: Richard Weinberger , linux-mm@kvack.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, upstream+pagemap@sigma-star.at, adobriyan@gmail.com, wangkefeng.wang@huawei.com, ryan.roberts@arm.com, hughd@google.com, peterx@redhat.com, avagin@google.com, lstoakes@gmail.com, vbabka@suse.cz, akpm@linux-foundation.org, usama.anjum@collabora.com, corbet@lwn.net References: <20240306232339.29659-1-richard@nod.at> <20240306232339.29659-2-richard@nod.at> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > --- > 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