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Date:   Sat, 10 Apr 2021 20:39:55 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Ilias Apalodimas <ilias.apalodimas@linaro.org>
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Subject: Re: [PATCH net-next v3 2/5] mm: add a signature in struct page
Message-ID: <20210410193955.GA2531743@casper.infradead.org>
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 <20210410154824.GZ2531743@casper.infradead.org>
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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/