linux-kernel - Re: Copy-on-write

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Message-ID: <3e8340490903262235g6174c6b4t4bfd76311be91eeb@mail.gmail.com>
Date:	Fri, 27 Mar 2009 01:35:18 -0400
From:	Bryan Donlan <bdonlan@...il.com>
To:	sidc7 <siddhartha.chhabra@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Copy-on-write

On Fri, Mar 27, 2009 at 1:15 AM, sidc7 <siddhartha.chhabra@...il.com> wrote:
>
> When the kernel does a COW, say from a "src" to a "dest" page, does it need
> to map the "src" and "dest" page to its address space or the kernel can
> directly initiate the read from "src" and write to "dest" page ?

If the source and destination pages are not in high memory (exactly
where this boundary is depends on your architecture) they do not need
to be mapped before copying. See cow_user_page in mm/memory.c,
copy_user_highpage in include/linux/highmem.h and kmap_atomic in
arch/x86/mm/highmem_32.c (as well as implementations for other
architectures)

Note that on 64-bit platforms, generally there will be no high memory,
and so remappings will never be needed to carry out a COW.
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