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Date: Fri, 20 Apr 2001 13:13:57 +0100
From: "Stephen C. Tweedie" <sct@redhat.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org, Stephen Tweedie <sct@redhat.com>
Subject: Re: RFC: pageable kernel-segments
Message-ID: <20010420131357.B1444@redhat.com>
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In-Reply-To: <9bi53d$5n6$1@cesium.transmeta.com>; from hpa@zytor.com on Tue, Apr 17, 2001 at 12:21:17PM -0700
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Hi,

On Tue, Apr 17, 2001 at 12:21:17PM -0700, H. Peter Anvin wrote:

> > Certain parts of drivers could get the __pageable prefix or so
> > (like the __init parts of drivers which get removed) for letting
> > the paging-code know that it can be discared if memory-pressure
> > demands it.
> 
> VMS does this.  It at least used to have a great tendency to crash
> itself, because it swapped out something that was called from a driver
> that was called by the swapper -- resulting in deadlock.  You need
> iron discipline for this to work right in all circumstances.

Actually, VMS doesn't do this, precisely because it is so hard to get
right.  VMS has both paged and non-paged pools for dynamically
allocated kernel memory, but the kernel code itself is non-pageable.  

The big problem with such pageable memory isn't really device driver
deadlocks --- the easy rule which makes that work is simply never to
use paged pool from a driver which might be involved in swapping. :)
Even more tricky is the handling of kernel locking --- you cannot
access any paged memory with a spinlock held unless you have pinned
the pages in core beforehand.

--Stephen
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