Re: [PATCH net-next v7 1/5] mm: add a signature in struct page

From: Matthew Wilcox <willy@infradead.org>
To: Matteo Croce <mcroce@linux.microsoft.com>
Cc: netdev@vger.kernel.org, linux-mm@kvack.org,
	Ayush Sawal <ayush.sawal@chelsio.com>,
	Vinay Kumar Yadav <vinay.yadav@chelsio.com>,
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	Tariq Toukan <tariqt@nvidia.com>,
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	David Ahern <dsahern@gmail.com>,
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	Saeed Mahameed <saeedm@nvidia.com>, Andrew Lunn <andrew@lunn.ch>,
	Paolo Abeni <pabeni@redhat.com>,
	Sven Auhagen <sven.auhagen@voleatech.de>
Subject: Re: [PATCH net-next v7 1/5] mm: add a signature in struct page
Date: Mon, 7 Jun 2021 14:52:38 +0100	[thread overview]
Message-ID: <YL4kpntfzMBXGSfV@casper.infradead.org> (raw)
In-Reply-To: <CAFnufp1e893Yz+KTjDvX4tyA8ngqmnMVudf1v0cBPdi9d_2zLw@mail.gmail.com>

On Sun, Jun 06, 2021 at 03:50:54AM +0200, Matteo Croce wrote:
> And change all the *_pfmemalloc functions to use page->lru.next like this?
> 
> @@ -1668,10 +1668,12 @@ struct address_space *page_mapping(struct page *page);
> static inline bool page_is_pfmemalloc(const struct page *page)
> {
>        /*
> -        * Page index cannot be this large so this must be
> -        * a pfmemalloc page.
> +        * This is not a tail page; compound_head of a head page is unused
> +        * at return from the page allocator, and will be overwritten
> +        * by callers who do not care whether the page came from the
> +        * reserves.
>         */

The comment doesn't make a lot of sense if we're switching to use
lru.next.  How about:

	/*
	 * lru.next has bit 1 set if the page is allocated from the
	 * pfmemalloc reserves.  Callers may simply overwrite it if
	 * they do not need to preserve that information.
	 */