Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

[joro@xxxxxxxxxx]

On Thu, Apr 26, 2018 at 05:49:58PM +0000, Kani, Toshi wrote:
> On Thu, 2018-04-26 at 19:23 +0200, joro@xxxxxxxxxx wrote:
> > So the PMD entry you clear can still be in a page-walk cache and this
> > needs to be flushed too before you can free the PTE page. Otherwise
> > page-walks might still go to the page you just freed. That is especially
> > bad when the page is already reallocated and filled with other data.
> 
> I do not understand why we need to flush processor caches here. x86
> processor caches are coherent with MESI.  So, clearing an PMD entry
> modifies a cache entry on the processor associated with the address,
> which in turn invalidates all stale cache entries on other processors.

A page walk cache is not about the processors data cache, its a cache
similar to the TLB to speed up page-walks by caching intermediate
results of previous page walks.


Thanks,

	Joerg