linux-kernel - Re: [PATCH net-next v5 1/5] mm: add a signature in struct page

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Date:   Fri, 14 May 2021 02:00:29 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     netdev@...r.kernel.org, linux-mm@...ck.org,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
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        "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>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
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        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v5 1/5] mm: add a signature in struct page

On Thu, May 13, 2021 at 06:58:42PM +0200, Matteo Croce wrote:
>  		struct {	/* page_pool used by netstack */
> +			/**
> +			 * @pp_magic: magic value to avoid recycling non
> +			 * page_pool allocated pages.
> +			 * It aliases with page->lru.next

I'm not really keen on documenting what aliases with what.
pp_magic also aliases with compound_head, 'next' (for slab),
and dev_pagemap.  This is an O(n^2) documentation problem ...

I feel like I want to document the pfmemalloc bit in mm_types.h,
but I don't have a concrete suggestion yet.

> +++ b/include/net/page_pool.h
> @@ -63,6 +63,8 @@
>   */
>  #define PP_ALLOC_CACHE_SIZE	128
>  #define PP_ALLOC_CACHE_REFILL	64
> +#define PP_SIGNATURE		(POISON_POINTER_DELTA + 0x40)

I wonder if this wouldn't be better in linux/poison.h?