linux-kernel - Re: why does the macro "ZERO

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Message-ID: <46684894.5010506@zytor.com>
Date:	Thu, 07 Jun 2007 11:04:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	"Robert P. J. Day" <rpjday@...dspring.com>,
	Satyam Sharma <satyam.sharma@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why does the macro "ZERO_PAGE" take an argument?

Ralf Baechle wrote:
> Cache aliases.  When the same page of physical memory is mapped twice to
> user space, let's say at address addr and addr + PAGE_SIZE this is normally
> harmless although wasteful on processors with virtually indexed caches as
> long as the page is mapped read-only such as in case of ZERO_PAGE.
> 
> If the same thing happens with a writable page there is the chance of
> severe data corruption.  Some members of the R4000 family are now trying
> to be helpful by throwing the kernel a "virtual coherency" exception.  The
> bad news about this exception is there might be thousands (the theoretical
> worst case would be millions) of it in a single second, so servicing can be
> very expensive.  For the ZERO page this can be avoided by using several
> pages mapped in a way such that their addresses don't conflict.

Note that it can be a *very expensive* waste even on machines that do
this in hardware.  Colouring the zeropage can have sizable performance
advantages for virtually no cost.

	-hpa

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