How is vmap_area inserted and can map_areas overlap?

[Peng Gao]

Replace miss typing va->va_start > tmp_va->va_start and va->va_end < tmp_va->va_end
with va->va_start > tmp_va->va_end and va->va_end < tmp_va->va_end.
It ensures va and tmp never overlap.

> On Sep 4, 2016, at 9:29 PM, Peng Gao <peng.gao.dut at gmail.com> wrote:
> 
> I am browsing the code 
> 
> static void __insert_vmap_area(struct vmap_area *va)
> {
> 	struct rb_node **p = &vmap_area_root.rb_node;
> 	struct rb_node *parent = NULL;
> 	struct rb_node *tmp;
> 
> 	while (*p) {
> 		struct vmap_area *tmp_va;
> 
> 		parent = *p;
> 		tmp_va = rb_entry(parent, struct vmap_area, rb_node);
> 		if (va->va_start < tmp_va->va_end)
> 			p = &(*p)->rb_left;
> 		else if (va->va_end > tmp_va->va_start)
> 			p = &(*p)->rb_right;
> 		else
> 			BUG();
> 	}
> 
> 	rb_link_node(&va->rb_node, parent, p);
> 	rb_insert_color(&va->rb_node, &vmap_area_root);
> 
> 	/* address-sort this list */
> 	tmp = rb_prev(&va->rb_node);
> 	if (tmp) {
> 		struct vmap_area *prev;
> 		prev = rb_entry(tmp, struct vmap_area, rb_node);
> 		list_add_rcu(&va->list, &prev->list);
> 	} else
> 		list_add_rcu(&va->list, &vmap_area_list);
> }
> 
> As far as I can know map_area is a range for the vmalloc to alloc non-contigious memory, which confuses me is that why not use va->va_start > tmp_va->va_start and va->va_end < tmp_va->va_end to insert a new vmap_area. Can a virtual address mapping overlap?
> 
> One more question
> 
> In alloc_map_area, I found this part of code.
> 
> 	/* from the starting point, walk areas until a suitable hole is found */
> 	while (addr + size > first->va_start && addr + size <= vend) {
> 		if (addr + cached_hole_size < first->va_start) /* cached_hole_size */
> 			cached_hole_size = first->va_start - addr;
> 		addr = ALIGN(first->va_end, align);
> 		if (addr + size < addr)
> 			goto overflow;
> 
> 		if (list_is_last(&first->list, &vmap_area_list))
> 			goto found;
> 
> 		first = list_next_entry(first, list);
> 	}
> 
> It seems like just inert a map_area at the end of list, what dose suitable hole means? 
> Why not add this?
> 
> if(addr+size < first->va_start) 
> 	goto notound 
>