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Date:   Tue, 6 Jun 2023 16:08:34 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Peter Xu <peterx@redhat.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH 05/12] powerpc: add pte_free_defer() for pgtables sharing
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On Tue, Jun 06, 2023 at 03:03:31PM -0400, Peter Xu wrote:
> 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.

Er yes, I botched that, the dtor and the free_page should be in a the
rcu callback function

Jason