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

From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mike Rapoport <rppt@kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	David Hildenbrand <david@redhat.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Qi Zheng <zhengqi.arch@bytedance.com>,
	Yang Shi <shy828301@gmail.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
	Alistair Popple <apopple@nvidia.com>,
	Ralph Campbell <rcampbell@nvidia.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Steven Price <steven.price@arm.com>,
	SeongJae Park <sj@kernel.org>,
	Naoya Horiguchi <naoya.horiguchi@nec.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
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	Axel Rasmussen <axelrasmussen@google.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Minchan Kim <minchan@kernel.org>,
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	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Jann Horn <jannh@google.com>,
	linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 05/12] powerpc: add pte_free_defer() for pgtables sharing page
Date: Tue, 6 Jun 2023 15:03:31 -0400	[thread overview]
Message-ID: <ZH+DAxLhIYpTlIFc@x1n> (raw)
In-Reply-To: <ZH95oobIqN0WO5MK@ziepe.ca>

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

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