Re: [PATCH 09/12] KVM: MMU: introduce pte-list lockless walker

[Gleb Natapov <gleb@redhat.com>]

On Thu, Aug 29, 2013 at 08:02:30PM +0800, Xiao Guangrong wrote:
> On 08/29/2013 07:33 PM, Xiao Guangrong wrote:
> > On 08/29/2013 05:31 PM, Gleb Natapov wrote:
> >> On Thu, Aug 29, 2013 at 02:50:51PM +0800, Xiao Guangrong wrote:
> >>> After more thinking, I still think rcu_assign_pointer() is unneeded when a entry
> >>> is removed. The remove-API does not care the order between unlink the entry and
> >>> the changes to its fields. It is the caller's responsibility:
> >>> - in the case of rcuhlist, the caller uses call_rcu()/synchronize_rcu(), etc to
> >>>   enforce all lookups exit and the later change on that entry is invisible to the
> >>>   lookups.
> >>>
> >>> - In the case of rculist_nulls, it seems refcounter is used to guarantee the order
> >>>   (see the example from Documentation/RCU/rculist_nulls.txt).
> >>>
> >>> - In our case, we allow the lookup to see the deleted desc even if it is in slab cache
> >>>   or its is initialized or it is re-added.
> >>>
> >> BTW is it a good idea? We can access deleted desc while it is allocated
> >> and initialized to zero by kmem_cache_zalloc(), are we sure we cannot
> >> see partially initialized desc->sptes[] entry? On related note what about
> >> 32 bit systems, they do not have atomic access to desc->sptes[].
> 
> Ah... wait. desc is a array of pointers:
> 
> struct pte_list_desc {
> 	u64 *sptes[PTE_LIST_EXT];
> 	struct pte_list_desc *more;
> };
> 
Yep, I misread it to be u64 bits and wondered why do we use u64 to store
pointers.

> assigning a pointer is aways aotomic, but we should carefully initialize it
> as you said. I will introduce a constructor for desc slab cache which initialize
> the struct like this:
> 
> for (i = 0; i < PTE_LIST_EXT; i++)
> 	desc->sptes[i] = NULL;
> 
> It is okay.
> 
I hope slab does not write anything into allocated memory internally if
constructor is present. BTW do you know what happens when SLAB debug is enabled
and SLAB_DESTROY_BY_RCU is set? Does poison value is written into freed
object (freed to slab, but not yet to page allocator)?

--
			Gleb.