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Subject: Re: [PATCH 7/8] mm: Document how to use struct page
References: <20171216164425.8703-1-willy@infradead.org>
 <20171216164425.8703-8-willy@infradead.org>
From: Randy Dunlap <rdunlap@infradead.org>
Message-ID: <4d963b8f-0010-fd20-013e-f53f27c8a7ce@infradead.org>
Date: Sat, 16 Dec 2017 09:47:16 -0800
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To: Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Christoph Lameter <cl@linux.com>, Matthew Wilcox <mawilcox@microsoft.com>

On 12/16/2017 08:44 AM, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> Be really explicit about what bits / bytes are reserved for users that
> want to store extra information about the pages they allocate.
> 
> Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> ---
>  include/linux/mm_types.h | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 1a3ba1f1605d..a517d210f177 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -31,7 +31,28 @@ struct hmm;
>   * it to keep track of whatever it is we are using the page for at the
>   * moment. Note that we have no way to track which tasks are using
>   * a page, though if it is a pagecache page, rmap structures can tell us
> - * who is mapping it.
> + * who is mapping it. If you allocate the page using alloc_pages(), you
> + * can use some of the space in struct page for your own purposes.
> + *
> + * Pages that were once in the page cache may be found under the RCU lock
> + * even after they have been recycled to a different purpose.  The page cache
> + * will read and writes some of the fields in struct page to lock the page,

"will read and writes" seems awkward to me.
Can that be:
    * reads and writes

> + * then check that it's still in the page cache.  It is vital that all users
> + * of struct page:
ta.
-- 
~Randy

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