From owner-freebsd-fs  Sat Jun  9 15: 1:53 2001
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Date: Sat, 9 Jun 2001 18:01:12 -0400
From: Alfred Perlstein <bright@rush.net>
To: Matt Dillon <dillon@earth.backplane.com>
Cc: Peter Wemm <peter@wemm.org>, tlambert2@mindspring.com,
	"Patrick W. Penzias Dirks" <pwd@apple.com>, FreeBSD-FS@FreeBSD.ORG,
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Subject: Re: Support for pivot_root-like system call?
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References: <3B2096AB.309B2D14@mindspring.com> <20010608153011.D7AF1380C@overcee.netplex.com.au> <20010608130128.E1832@superconductor.rush.net> <200106091651.f59GpgF07875@earth.backplane.com>
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* Matt Dillon <dillon@earth.backplane.com> [010609 12:51] wrote:
> 
> :The cache coherency bugs are not fixed for all cases, being able to
> :ask the underlying filesystem for the vm_object does not solve the
> :problem.
> :
> :What if i have a stacking layer where each page alternates
> :between two files that i'm stacked on top of?
> :
> :Passing the vm_object back doesn't work for this.  Unless something
> :else has been done.
> :
> :-Alfred Perlstein [alfred@freebsd.org]
> 
>     Well, I suppose a vm_map would solve this, though for the specific
>     alternating case (or a RAID configuration of some sort) it would be
>     really expensive to do with a vm_map.  We would need some kind of
>     synthesized translation capability in the vm_map to make it efficient.

I think the proper method would be to use GET/PUTPAGES, where a GETPAGES
would require a subsequent PUT/UNLOCKPAGES.

The problem is that a bunch of code digs into the backing data of
the vm and vfs structures without bothering to query the vm/vfs and
ask if it's ok (besideds respecting locks, and even that's not always
true).

-- 
-Alfred Perlstein [alfred@freebsd.org]
Instead of asking why a piece of software is using "1970s technology,"
start asking why software is ignoring 30 years of accumulated wisdom.

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