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

[Peter Xu]

On Tue, Jun 06, 2023 at 03:23:30PM -0300, Jason Gunthorpe wrote:
> On Mon, Jun 05, 2023 at 08:40:01PM -0700, Hugh Dickins wrote:
> 
> > diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> > index 20652daa1d7e..e4f58c5fc2ac 100644
> > --- a/arch/powerpc/mm/pgtable-frag.c
> > +++ b/arch/powerpc/mm/pgtable-frag.c
> > @@ -120,3 +120,54 @@ void pte_fragment_free(unsigned long *table, int kernel)
> >  		__free_page(page);
> >  	}
> >  }
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +#define PTE_FREE_DEFERRED 0x10000 /* beyond any PTE_FRAG_NR */
> > +
> > +static void pte_free_now(struct rcu_head *head)
> > +{
> > +	struct page *page;
> > +	int refcount;
> > +
> > +	page = container_of(head, struct page, rcu_head);
> > +	refcount = atomic_sub_return(PTE_FREE_DEFERRED - 1,
> > +				     &page->pt_frag_refcount);
> > +	if (refcount < PTE_FREE_DEFERRED) {
> > +		pte_fragment_free((unsigned long *)page_address(page), 0);
> > +		return;
> > +	}
> 
> From what I can tell power doesn't recycle the sub fragment into any
> kind of free list. It just waits for the last fragment to be unused
> and then frees the whole page.
> 
> So why not simply go into pte_fragment_free() and do the call_rcu directly:
> 
> 	BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
> 	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> 		if (!kernel)
> 			pgtable_pte_page_dtor(page);
> 		call_rcu(&page->rcu_head, free_page_rcu)

We need to be careful on the lock being freed in pgtable_pte_page_dtor(),
in Hugh's series IIUC we need the spinlock being there for the rcu section
alongside the page itself.  So even if to do so we'll need to also rcu call 
pgtable_pte_page_dtor() when needed.

-- 
Peter Xu