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Date: Tue, 02 Dec 2008 18:52:49 -0800
From: Kris Kennaway <kris@FreeBSD.org>
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To: Mike Tancsa <mike@sentex.net>
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Cc: Kostik Belousov <kostikbel@gmail.com>, freebsd-stable@freebsd.org
Subject: Re: repeatable crash on RELENG7
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Mike Tancsa wrote:
> At 09:20 AM 12/2/2008, Kostik Belousov wrote:
>> On Tue, Dec 02, 2008 at 09:12:54AM -0500, Mike Tancsa wrote:
>> > At 08:38 AM 12/2/2008, Kostik Belousov wrote:
>> > >>
>> > >> mdconfig -a -t malloc -s 1800M
>> > >You cannot have ~ 2Gb of kernel memory allocated for md, at least 
>> not on
>> > >i386.
>> >
>> > Thanks,  how do I find out what the limit is on a machine ? Is it
>> > vm.kvm_size ?
>>
>> It is much less, and highly depends on your load, since KVA is used 
>> for all
>> kind of allocations made by kernel. I think either md(4) or 
>> mdconfig(8) have
>> a warning about malloc backing for md.
> 
> Thanks!  A warning might be helpful to prevent such foot shooting :)

              malloc   Storage for this type of memory disk is allocated 
with
                       malloc(9).  This limits the size to the malloc bucket
                       limit in the kernel.  If the -o reserve option is not
                       set, creating and filling a large malloc-backed 
memory
                       disk is a very easy way to panic a system.

You almost never want to use malloc backing for a md, in favour of swap 
backing.

Kris