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@xxxxxxxxxx/ 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
