[PATCH v2 05/12] powerpc: add pte_free_defer() for pgtables sharing page

[Hugh Dickins]

On Tue, 20 Jun 2023, Jason Gunthorpe wrote:
> On Tue, Jun 20, 2023 at 12:47:54AM -0700, Hugh Dickins wrote:
> > Add powerpc-specific pte_free_defer(), to call pte_free() via call_rcu().
> > pte_free_defer() will be called inside khugepaged's retract_page_tables()
> > loop, where allocating extra memory cannot be relied upon.  This precedes
> > the generic version to avoid build breakage from incompatible pgtable_t.
> > 
> > This is awkward because the struct page contains only one rcu_head, but
> > that page may be shared between PTE_FRAG_NR pagetables, each wanting to
> > use the rcu_head at the same time: account concurrent deferrals with a
> > heightened refcount, only the first making use of the rcu_head, but
> > re-deferring if more deferrals arrived during its grace period.
> 
> You didn't answer my question why we can't just move the rcu to the
> actual free page?

I thought that I had answered it, perhaps not to your satisfaction:

https://lore.kernel.org/linux-mm/9130acb-193-6fdd-f8df-75766e663978@google.com/

My conclusion then was:
Not very good reasons: good enough, or can you supply a better patch?

Hugh

> 
> Since PPC doesn't recycle the frags, we don't need to carefully RCU
> free each frag, we just need to RCU free the entire page when it
> becomes eventually free?
> 
> Jason