Issue with max_threads (and other resources) and highmem

[Dave McCracken <dmccr@us.ibm.com>]

I recently had pointed out to me that the default value for max_threads (ie
the max number of tasks per system) doesn't work right on machines with
lots of memory.

A quick examination of fork_init() shows that max_threads is supposed to be
limited so its stack/task_struct takes no more than half of physical
memory.  This calculation ignores the fact that task_structs must be
allocated from the normal pool and not the highmem pool, which is a clear
bug.  On a machine with enough physical memory it's possible for all of
normal memory to be allocated to task_structs, which tends to make the
machine die.  

fork_init() gets its knowledge of physical memory passed in from
start_kernel(), which sets it from mum_physpages.  This parameter is also
passed to several other init functions.

My question boils down to this...  Should we change start_kernel() to limit
the physical memory size it passes to the init functions to not include
high memory, or should we only do it for fork_init()?  What is the best way
to do calculate this number?  I don't see any simple way in
architecture-independent code to get the size of high memory vs normal
memory.

What's the best approach here?

Thanks,
Dave McCracken

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Dave McCracken          IBM Linux Base Kernel Team      1-512-838-3059
dmccr@us.ibm.com                                        T/L   678-3059