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

[Jason Gunthorpe]

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?

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