linux-kernel - RE: [PATCH 2/2] mm: Indicate pfmemalloc pages in compound

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Date:   Sat, 17 Apr 2021 21:13:45 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Matthew Wilcox (Oracle)'" <willy@...radead.org>,
        "brouer@...hat.com" <brouer@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "mcroce@...ux.microsoft.com" <mcroce@...ux.microsoft.com>,
        "grygorii.strashko@...com" <grygorii.strashko@...com>,
        "arnd@...nel.org" <arnd@...nel.org>, "hch@....de" <hch@....de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "mgorman@...e.de" <mgorman@...e.de>
Subject: RE: [PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head

From: Matthew Wilcox (Oracle) <willy@...radead.org>
> Sent: 17 April 2021 00:07
> 
> The net page_pool wants to use a magic value to identify page pool pages.
> The best place to put it is in the first word where it can be clearly a
> non-pointer value.  That means shifting dma_addr up to alias with ->index,
> which means we need to find another way to indicate page_is_pfmemalloc().
> Since page_pool doesn't want to set its magic value on pages which are
> pfmemalloc, we can use bit 1 of compound_head to indicate that the page
> came from the memory reserves.
> 
...
>  		struct {	/* page_pool used by netstack */
> -			/**
> -			 * @dma_addr: might require a 64-bit value on
> -			 * 32-bit architectures.
> -			 */
> +			unsigned long pp_magic;
> +			unsigned long xmi;
> +			unsigned long _pp_mapping_pad;
>  			unsigned long dma_addr[2];
>  		};

You've deleted the comment.

I also think there should be a comment that dma_addr[0]
must be aliased to ->index.
(Or whatever all the exact requirements are.)

	David

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