Re: [PATCH] rmaplock 2/6 SLAB_DESTROY_BY_RCU

[Manfred Spraul]

Hugh Dickins wrote:

> It's okay to take anon_vma->lock after it's freed, so long as it remains
> a struct anon_vma (its list would become empty, or perhaps reused for an
> unrelated anon_vma: but no problem since we always check that the page
> located is the right one); but corruption if that memory gets reused for
> some other purpose.
>
>  
An interesting idea:
The slab caches are object caches. If a rcu user only needs a valid 
object but doesn't care which one then there is no need to wait for a 
quiescent cycle after free - the quiescent cycle can be delayed until 
the destructor is called.

But there are two flaws in your patch:
- you must disable poisoning and unmapping if SLAB_DESTROY_BY_RCU is set.
- either delay the dtor calls a well or fail if an object has a non-NULL 
dtor and SLAB_DESTROY_BY_RCU is set.

--
   Manfred
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