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To: linux-kernel@vger.kernel.org
From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: Security question: "Text file busy" overwriting executables but
 not shared libraries?
Date: Sat, 13 Oct 2001 22:27:30 +0000 (UTC)
Organization: Transmeta Corporation
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In article <Pine.LNX.4.33.0110131219520.8900-100000@penguin.transmeta.com>,
Linus Torvalds  <torvalds@transmeta.com> wrote:
>
>The explicit flag is probably a good idea also because of usage patterns
>(PAGE_COPY is a slowdown _if_ the file is actually written to or even
>mapped shared).

Actually, I missed the obvious case: quite often when you do a "read()",
the reader itself will end up writing to the area read into.  In which
case doing the PAGE_COPY would also slow down measurably, due to the
extra overhead of the copy-on-write fault (which not just does the copy
that we tried to avoid, but will take a fault and more VM locks). 

So if we want to do this optimization, we _definitely_ want it to be
explicitly controlled by a flag, like O_DIRECT is.  There are just too
many cases where it's a pessimization, and while the user can often tell
before-hand, the kernel simply cannot. 

		Linus
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