superpages and kmem on amd64

Marko Zec zec at
Sun May 20 22:48:58 UTC 2012

On Sunday 20 May 2012 19:34:26 Alan Cox wrote:
> > In any case, I wish to be certain that a particular kmem virtual address
> > range is mapped to superpages - how can I enforce that at malloc time,
> > and / or find out later if I really got my kmem mapped to superpages? 
> > Perhaps vm_map_lookup() could provide more info, but I'm wondering if
> > someone already wrote a wrapper function for that, which takes only the
> > base virtual address as a single argument?
> Try using pmap_mincore() to verify that the mappings are superpages.

flags = pmap_mincore(vmspace_pmap(curthread->td_proc->p_vmspace), 
(vm_offset_t) addr));

OK, that works, and now I know my kmem chunk is on a superpage, horray!!!  

> > BTW, apparently malloc(size, M_TEMP, M_NOWAIT) requests fail for size> 
> > 1G, even at boot time.  Any ideas how to circumvent that (8.3-STABLE,
> > amd64, 4G physical RAM)?
> I suspect that you need to increase the size of your kmem map.

Huh any hints how should I achieve that?  In desperation I placed


in /boot/loader.conf and got this:

vm.kmem_map_free: 8123924480
vm.kmem_map_size: 8364032
vm.kmem_size_scale: 1
vm.kmem_size_max: 329853485875
vm.kmem_size_min: 0
vm.kmem_size: 8132288512

but malloc(2G) still fails...



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