RE: savecore: Value too large to be stored in data type

From: Kris Kennaway [mailto:kris_at_obsecurity.org]
> On Thu, Mar 04, 2004 at 04:41:12PM -0500, Don Bowman wrote:
> > has anyone seen this problem on a panic?
> > 
> > i have 4GB of physical memory, running 5.2.1 RC.
> > I had a panic occur, and it won't write the whole vmcore.
> > 
> > 1985347584 is the size of the vmcore i get (not compressed).
> > Its not useful as far as i can tell, gdb doesn't know where
> > to begin.
> > 
> > My swap partition is 8GB. The filesystem i'm saving the
> > vmcore too is plenty big enough.
> 
> Can 8GB of swap even be addressed on i386?
> 

The below is excerpted from an email by matt dillon on july 6, 2002 to the
mailing list, and seems to indicate there should be no problem with 8GB of
swap:

From: Matthew Dillon Subject: Re: How does swap work address spacewise?
Date: Sat, 6 Jul 2002 16:34:18 -0700 (PDT) The nominal limit for swap space
is around 14 GB due to limitations in available KVM. There are three major
limiting factors in the kernel:

    * The swap bitmap eats 2 bits per page of swap.  The bitmap is sized
      to handle NSWAP (default 4) x size_of_largest_swap_partition.

    * The system has to keep track of pages that are swapped out.
      The system reserves 8 x  worth of
      KVM to keep track of swapped out pages.  A machine with 4G
      of ram reserves enough KVA to hold 32GB worth of swapped out pages.

    * The system limits the size of the above zone to VM_SWZONE_SIZE_MAX,
      which is typically around 70 MB of KVM (enough to hold 14 GB worth
      of swap mappings).

So the nominal limit is around 14 GB on a 32 bit architecture. With tuning
this limit can be bumped up, but the practical limit is going to be around
60GB unless you give the kernel more KVA (reducing the amount of VM a user
process can access). On 64 bit architectures the only real limitation is how
much physical memory one is willing to burn to hold active swap mappings.
And, no, there are no plans to swap out the swap mappings themselves... that
would be a complete waste of effort. -Matt