From: David Hildenbrand <david@redhat.com>
To: Richard Weinberger <richard@nod.at>
Cc: linux-mm <linux-mm@kvack.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Linux Doc Mailing List <linux-doc@vger.kernel.org>,
upstream+pagemap@sigma-star.at, adobriyan@gmail.com,
wangkefeng wang <wangkefeng.wang@huawei.com>,
ryan roberts <ryan.roberts@arm.com>,
hughd@google.com, peterx@redhat.com, avagin@google.com,
lstoakes@gmail.com, vbabka <vbabka@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
usama anjum <usama.anjum@collabora.com>,
Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit
Date: Thu, 7 Mar 2024 12:15:38 +0100 [thread overview]
Message-ID: <9ebc9faf-0bb0-4409-b93a-5beddfc4929b@redhat.com> (raw)
In-Reply-To: <861682210.23281.1709809857201.JavaMail.zimbra@nod.at>
On 07.03.24 12:10, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
>> Von: "David Hildenbrand" <david@redhat.com>
>> An: "richard" <richard@nod.at>, "linux-mm" <linux-mm@kvack.org>
>>> + 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.
>
> Can you please help me correct my obvious misunderstanding?
We'll perform a page copy of an anonymous page only if the page is not
detected as exclusive to the process.
So a better description could be:
"In an private mapping, having the writable bit clear can indicate that
next write access will result in copy-on-write during a page fault. Note
that exclusive anonymous pages can be mapped read-only, and they might
simply get remapped writable during the next write fault, avoiding a
page copy."
--
Cheers,
David / dhildenb
next prev parent reply other threads:[~2024-03-07 11:15 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-06 23:23 [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable Richard Weinberger
2024-03-06 23:23 ` [PATCH 2/2] [RFC] pagemap.rst: Document write bit Richard Weinberger
2024-03-07 10:52 ` David Hildenbrand
2024-03-07 11:10 ` Richard Weinberger
2024-03-07 11:15 ` David Hildenbrand [this message]
2024-03-10 22:14 ` Lorenzo Stoakes
2024-03-07 10:44 ` [PATCH 1/2] [RFC] proc: pagemap: Expose whether a PTE is writable Muhammad Usama Anjum
2024-03-07 10:52 ` David Hildenbrand
2024-03-07 11:10 ` Richard Weinberger
2024-03-07 11:20 ` David Hildenbrand
2024-03-07 11:51 ` Richard Weinberger
2024-03-07 11:59 ` David Hildenbrand
2024-03-07 12:09 ` David Hildenbrand
2024-03-07 14:42 ` Richard Weinberger
2024-03-10 21:55 ` Lorenzo Stoakes
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