Re: locking user space memory in kernel

[Manfred Spraul <manfred@colorfullife.com>]

Eli Cohen wrote:

> Roland Dreier wrote:
>
>> I don't think copying all the registered memory on fork() is feasible,
>> because it's going to kill performance (especially since exec() is
>> likely to immediately follow the fork() in the child).  Also, there
>> may not be enough memory around to copy everything.
>>
>>  
>>
> Suppose a new vma flag is introduced, VM_NOCOW and an API to apply 
> this flag on a range of addreses, splitting or unifying vmas as necessary.

Something like that. But it should be hidden within a suitable 
abstraction. get_user_pages and then put_page is not stateful enough. 
Actually it's fundamentally broken for platform that need cache flush 
calls. create_page_mapping/free_page_mapping, or something like that.

And I still think that the initial implementation should copy the 
affected pages within fork() - it might be slow, but at least it's 
simple and correct. _If_ it's too slow, then it can be fixed later.

--
    Manfred