Re: [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user_ram

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Oct 16, 2018, at 2:30 PM, rostedt rostedt@goodmis.org wrote:

> On Wed, 10 Oct 2018 15:19:24 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> + * vm_unmap_user_ram - unmap linear kernel address space set up by
>> vm_map_user_ram
>> + * @mem: the pointer returned by vm_map_user_ram
>> + * @count: the count passed to that vm_map_user_ram call (cannot unmap partial)
>> + */
>> +void vm_unmap_user_ram(const void *mem, unsigned int count)
>> +{
>> +	unsigned long size = (unsigned long)count << PAGE_SHIFT;
>> +	unsigned long addr = (unsigned long)mem;
>> +	struct vmap_area *va;
>> +
>> +	might_sleep();
>> +	BUG_ON(!addr);
>> +	BUG_ON(addr < VMALLOC_START);
>> +	BUG_ON(addr > VMALLOC_END);
>> +	BUG_ON(!PAGE_ALIGNED(addr));
>> +
>> +	debug_check_no_locks_freed(mem, size);
>> +	va = find_vmap_area(addr);
>> +	BUG_ON(!va);
>> +	free_unmap_vmap_area(va);
>> +}
>> +EXPORT_SYMBOL(vm_unmap_user_ram);
>> +
> 
> Noticing this from Sergey's question in another patch, why are you
> using BUG_ON()? That's rather extreme and something we are trying to
> avoid adding more of (I still need to remove the BUG_ON()s I've added
> over ten years ago). I don't see why all these BUG_ON's can't be turned
> into:
> 
>	if (WARN_ON(x))
>		return;

I borrowed the code from vm_unmap_ram(), which has the following checks:

        BUG_ON(!addr);
        BUG_ON(addr < VMALLOC_START);
        BUG_ON(addr > VMALLOC_END);
        BUG_ON(!PAGE_ALIGNED(addr));
[...]
        va = find_vmap_area(addr);
        BUG_ON(!va);

The expectation here is that inputs to vm_unmap_ram() should always come from
vm_map_ram(), so an erroneous input is an internal kernel bug. I applied the
same logic to vm_unmap_user_ram() and vm_map_user_ram().

Should we turn all those BUG_ON() into if (WARN_ON(x)) return; in vm_{map,unmap}_ram
as well ?

Thanks,

Mathieu


> 
> -- Steve

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com