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Date: Fri, 12 Apr 2002 17:08:42 -0400
From: Jake Burkholder <jake@locore.ca>
To: Alan Cox <alc@cs.rice.edu>
Cc: Thomas Moestl <tmm@FreeBSD.org>, cvs-committers@FreeBSD.org,
	cvs-all@FreeBSD.org
Subject: Re: cvs commit: src/sys/kern sys_pipe.c
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References: <200204121938.g3CJcfk69260@freefall.freebsd.org> <Pine.GSO.4.33.0204121536490.11123-100000@cs.rice.edu>
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Apparently, On Fri, Apr 12, 2002 at 03:42:37PM -0500,
	Alan Cox said words to the effect of;

> 
> 
> On Fri, 12 Apr 2002, Thomas Moestl wrote:
> 
> > tmm         2002/04/12 12:38:41 PDT
> >
> >   Modified files:
> >     sys/kern             sys_pipe.c
> >   Log:
> >   Do not use pmap_kextract() to find out the physical address of a user
> >   belong to a user virtual address; while this happens to work on some
> >   architectures, it can't on sparc64, since user and kernel virtual
> >   address spaces overlap there (the distinction between them is done via
> >   separate address space identifiers).
> >
> 
> Why not pmap_extract() on the map->pmap?  pmap_extract() is the equivalent
> of pmap_kextract() for ordinary pmap's, i.e., not the kernel pmap.
> 
> Alan

Because the sparc64 pmap uses a hash table like structure for non-kernel
pmaps with fixed size buckets.  If there are too many hash collisions a
mapping can be forced out causing a replacement and a soft fault on the
next access.  Basically, pmap_extract is not guaranteed to work for ordinary
pmaps.

Jake

> 
> >   Instead, look up the page in the vm_map of the process in question.
> >
> >   Reviewed by:    jake
> >
> >   Revision  Changes    Path
> >   1.102     +17 -6     src/sys/kern/sys_pipe.c
> >

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