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

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Date:   Tue, 11 May 2021 15:18:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc:     Matteo Croce <mcroce@...ux.microsoft.com>, netdev@...r.kernel.org,
        linux-mm@...ck.org, Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>,
        Will Deacon <will@...nel.org>,
        Michel Lespinasse <walken@...gle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Roman Gushchin <guro@...com>, Hugh Dickins <hughd@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Cong Wang <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        bpf@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v4 1/4] mm: add a signature in struct page

On Tue, May 11, 2021 at 05:11:13PM +0300, Ilias Apalodimas wrote:
> 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?

I'd rather you didn't use page_private ... Any reason not to use:

			unsigned long pp_magic;
			struct page_pool *pp;
			unsigned long _pp_mapping_pad;
			unsigned long dma_addr[2];

?