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

From: Matthew Wilcox <willy@infradead.org>
To: Matteo Croce <mcroce@linux.microsoft.com>
Cc: netdev@vger.kernel.org, linux-mm@kvack.org,
	Ayush Sawal <ayush.sawal@chelsio.com>,
	Vinay Kumar Yadav <vinay.yadav@chelsio.com>,
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	Jesper Dangaard Brouer <hawk@kernel.org>,
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	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 14:45:32 +0100	[thread overview]
Message-ID: <YJqKfNh6l3yY2daM@casper.infradead.org> (raw)
In-Reply-To: <20210511133118.15012-2-mcroce@linux.microsoft.com>

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.