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Date:   Sun, 11 Apr 2021 12:05:17 +0200
From:   Jesper Dangaard Brouer <brouer@redhat.com>
To:     Matthew Wilcox <willy@infradead.org>
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Subject: Re: [PATCH net-next v3 2/5] mm: add a signature in struct page
Message-ID: <20210411120500.73c1cadb@carbon>
In-Reply-To: <20210410193955.GA2531743@casper.infradead.org>
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On Sat, 10 Apr 2021 20:39:55 +0100
Matthew Wilcox <willy@infradead.org> wrote:

> On Sat, Apr 10, 2021 at 09:27:31PM +0300, Ilias Apalodimas wrote:
> > > Can this page_pool be used for TCP RX zerocopy? If yes then PageType
> > > can not be used.  
> > 
> > Yes it can, since it's going to be used as your default allocator for
> > payloads, which might end up on an SKB.
> > So we have to keep the extra added field on struct page for our mark.
> > Matthew had an intersting idea.  He suggested keeping it, but changing the 
> > magic number, so it can't be a kernel address, but I'll let him follow 
> > up on the details.  
> 
> Sure!  So, given the misalignment problem I discovered yesterday [1],
> we probably want a page_pool page to look like:
> 
> unsigned long	flags;
> unsigned long	pp_magic;
> unsigned long	xmi;
> unsigned long	_pp_mapping_pad;
> dma_addr_t	dma_addr;	/* might be one or two words */
> 
> The only real restriction here is that pp_magic should not be a valid
> pointer, and it must have the bottom bit clear.  I'd recommend something
> like:
> 
> #define PP_MAGIC	(0x20 + POISON_POINTER_DELTA)
> 
> This leaves page->mapping as NULL, so you don't have to worry about
> clearing it before free.
>
> [1] https://lore.kernel.org/linux-mm/20210410024313.GX2531743@casper.infradead.org/

I didn't see this, before asking[2] for explaining your intent.
I still worry about page->index, see [2].

[2] https://lore.kernel.org/netdev/20210411114307.5087f958@carbon/

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer