Re: [PATCH net-next v4 1/4] mm: add a signature in struct page

From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Matteo Croce <mcroce@linux.microsoft.com>,
	netdev@vger.kernel.org, linux-mm@kvack.org,
	Ayush Sawal <ayush.sawal@chelsio.com>,
	Vinay Kumar Yadav <vinay.yadav@chelsio.com>,
	Rohit Maheshwari <rohitm@chelsio.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	Marcin Wojtas <mw@semihalf.com>,
	Russell King <linux@armlinux.org.uk>,
	Mirko Lindner <mlindner@marvell.com>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Tariq Toukan <tariqt@nvidia.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	John Fastabend <john.fastabend@gmail.com>,
	Boris Pismenny <borisp@nvidia.com>, Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Yu Zhao <yuzhao@google.com>,
	Will Deacon <will@kernel.org>,
	Michel Lespinasse <walken@google.com>,
	Fenghua Yu <fenghua.yu@intel.com>, Roman Gushchin <guro@fb.com>,
	Hugh Dickins <hughd@google.com>, Peter Xu <peterx@redhat.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Alexander Lobakin <alobakin@pm.me>,
	Cong Wang <cong.wang@bytedance.com>, wenxu <wenxu@ucloud.cn>,
	Kevin Hao <haokexin@gmail.com>,
	Jakub Sitnicki <jakub@cloudflare.com>,
	Marco Elver <elver@google.com>,
	Willem de Bruijn <willemb@google.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Yunsheng Lin <linyunsheng@huawei.com>,
	Guillaume Nault <gnault@redhat.com>,
	linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org,
	bpf@vger.kernel.org, Eric Dumazet <edumazet@google.com>,
	David Ahern <dsahern@gmail.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	Saeed Mahameed <saeedm@nvidia.com>, Andrew Lunn <andrew@lunn.ch>,
	Paolo Abeni <pabeni@redhat.com>,
	Sven Auhagen <sven.auhagen@voleatech.de>
Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page
Date: Tue, 11 May 2021 17:11:13 +0300	[thread overview]
Message-ID: <YJqQgYSWH2qan1GS@apalos.home> (raw)
In-Reply-To: <YJqKfNh6l3yY2daM@casper.infradead.org>

Hi Matthew,

On Tue, May 11, 2021 at 02:45:32PM +0100, Matthew Wilcox wrote:
> On Tue, May 11, 2021 at 03:31:15PM +0200, Matteo Croce wrote:
> > @@ -101,6 +101,7 @@ struct page {
> >  			 * 32-bit architectures.
> >  			 */
> >  			unsigned long dma_addr[2];
> > +			unsigned long signature;
> >  		};
> >  		struct {	/* slab, slob and slub */
> >  			union {
> 
> No.  Signature now aliases with page->mapping, which is going to go
> badly wrong for drivers which map this page into userspace.
> 
> I had this as:
> 
> +                       unsigned long pp_magic;
> +                       unsigned long xmi;
> +                       unsigned long _pp_mapping_pad;
>                         unsigned long dma_addr[2];
> 
> and pp_magic needs to be set to something with bits 0&1 clear and
> clearly isn't a pointer.  I went with POISON_POINTER_DELTA + 0x40.

Regardless to the changes required, there's another thing we'd like your
opinion on.
There was a change wrt to the previous patchset. We used to store the
struct xdp_mem_info into page->private.  On the new version we store the
page_pool ptr address in page->private (there's an explanation why on the
mail thread, but the tl;dr is that we can get some more speed and keeping
xdp_mem_info is not that crucial). So since we can just store the page_pool
address directly, should we keep using page->private or it's better to
do: 

+                       unsigned long pp_magic;
+                       unsigned long pp_ptr;
+                       unsigned long _pp_mapping_pad;
                        unsigned long dma_addr[2];
and use pp_ptr?

Thanks
/Ilias