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Date:   Tue, 11 May 2021 14:45:32 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Matteo Croce <mcroce@linux.microsoft.com>
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Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page
Message-ID: <YJqKfNh6l3yY2daM@casper.infradead.org>
References: <20210511133118.15012-1-mcroce@linux.microsoft.com>
 <20210511133118.15012-2-mcroce@linux.microsoft.com>
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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.