Re: [RFC PATCH 3/4] mm/vmalloc.c: Allow lowmem to be tracked in vmalloc

[Dave Hansen <dave@sr71.net>]

On 11/11/2013 03:26 PM, Laura Abbott wrote:
> +config ENABLE_VMALLOC_SAVING
> +	bool "Intermix lowmem and vmalloc virtual space"
> +	depends on ARCH_TRACKS_VMALLOC
> +	help
> +	  Some memory layouts on embedded systems steal large amounts
> +	  of lowmem physical memory for purposes outside of the kernel.
> +	  Rather than waste the physical and virtual space, allow the
> +	  kernel to use the virtual space as vmalloc space.

I really don't think this needs to be exposed with help text and so
forth.   How about just defining a 'def_bool n' with some comments and
let the architecture 'select' it?

> +#ifdef ENABLE_VMALLOC_SAVING
> +int is_vmalloc_addr(const void *x)
> +{
> +	struct rb_node *n;
> +	struct vmap_area *va;
> +	int ret = 0;
> +
> +	spin_lock(&vmap_area_lock);
> +
> +	for (n = rb_first(vmap_area_root); n; rb_next(n)) {
> +		va = rb_entry(n, struct vmap_area, rb_node);
> +		if (x >= va->va_start && x < va->va_end) {
> +			ret = 1;
> +			break;
> +		}
> +	}
> +
> +	spin_unlock(&vmap_area_lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL(is_vmalloc_addr);
> +#endif

It's probably worth noting that this makes is_vmalloc_addr() a *LOT*
more expensive than it was before.  There are a couple dozen of these in
the tree in kinda weird places (ext4, netlink, tcp).  You didn't
mention it here, but you probably want to at least make sure you're not
adding a spinlock and a tree walk in some critical path.

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