From owner-cvs-all  Fri Apr 12 15:54: 4 2002
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Date: Fri, 12 Apr 2002 17:53:50 -0500 (CDT)
From: Alan Cox <alc@cs.rice.edu>
To: Jake Burkholder <jake@locore.ca>
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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On Fri, 12 Apr 2002, Jake Burkholder wrote:

> 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.
>

The previous line is a vm_fault_quick() that should trigger the soft
fault so that pmap_extract() returns a useful value.

Alan



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