linux-kernel - Re: Copy-on-write

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Date:	Thu, 26 Mar 2009 23:43:03 -0700 (PDT)
From:	sidc7 <siddhartha.chhabra@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: Copy-on-write


> 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.

Thanks for the quick reply. In general if the kernel wishes to read any
arbitrary page mapped to an application's address space, will the kernel
cause a page fault on that page, since the page is currently not in its
address space and the kernel is wishing to read from this page?

Thanks

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