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Date: Thu, 7 Mar 2024 12:15:38 +0100
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Subject: Re: [PATCH 2/2] [RFC] pagemap.rst: Document write bit
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To: Richard Weinberger <richard@nod.at>
Cc: linux-mm <linux-mm@kvack.org>,
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From: David Hildenbrand <david@redhat.com>
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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