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Re: [patch 0/5] Support for sanitization flag in low-level page allocator

From:  Peter Zijlstra <peterz-AT-infradead.org>
To:  Alan Cox <alan-AT-lxorguk.ukuu.org.uk>
Subject:  Re: [patch 0/5] Support for sanitization flag in low-level page allocator
Date:  Thu, 28 May 2009 21:44:54 +0200
Message-ID:  <1243539894.6645.85.camel@laptop>
Cc:  Ingo Molnar <mingo-AT-elte.hu>, Rik van Riel <riel-AT-redhat.com>, "Larry H." <research-AT-subreption.com>, linux-kernel-AT-vger.kernel.org, Linus Torvalds <torvalds-AT-osdl.org>, linux-mm-AT-kvack.org, Ingo Molnar <mingo-AT-redhat.com>, pageexec-AT-freemail.hu, Linus Torvalds <torvalds-AT-linux-foundation.org>
Archive-link:  Article

On Thu, 2009-05-28 at 12:50 +0100, Alan Cox wrote:
> The performance cost of such a security action are NIL when the feature
> is disabled. So the performance cost in the general case is irrelevant.

Not really, much of the code posted in this thread has the form:

int sanitize_all_mem; /* note the lack of __read_mostly */

void some_existing_function()
{
	if (sanitize_all_mem) { /* extra branch */
		/* do stuff */
	}
}

void sanitize_obj(void *obj)
{
	if (!sanitize_all_mem) /* extra branch */
		return;

	/* do stuff */
}


void another_existing_function()
{
	sanitize_obj(obj); /* extra call */
}

That doesn't equal NIL, that equals extra function calls and branches.

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