linux-kernel - Re: [PATCH v2 1/4] define PAGECACHE_TAG

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Date:	Fri, 4 Jul 2014 11:16:39 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Johannes Weiner <hannes@...xchg.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Miller <davem@...emloft.net>,
	Andres Freund <andres@...quadrant.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Dave Hansen <dave.hansen@...el.com>,
	Christoph Hellwig <hch@...radead.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v2 1/4] define PAGECACHE_TAG_* as enumeration under
 include/uapi

On Thu, Jul 03, 2014 at 05:52:12PM -0400, Naoya Horiguchi wrote:
> We need the pagecache tags to be exported to userspace later in this
> series for fincore(2), so this patch moves the definition to the new
> include file for preparation. We also use the number of pagecache tags,
> so this patch also adds it.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>

NACK.

The radix tree tags are deeply internal implementation details.
They are an artifact of the current mark-and-sweep writeback
algorithm, and as such should never, ever be exposed to userspace,
let alone fixed in an ABI we need to support forever more.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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