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Date: Fri, 4 Jul 2014 03:12:30 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Wu Fengguang <fengguang.wu@intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
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        Christoph Hellwig <hch@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        Naoya Horiguchi <nao.horiguchi@gmail.com>
Subject: Re: [PATCH v2 2/4] mm: introduce fincore()
Message-ID: <20140704101230.GA24688@infradead.org>
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On Thu, Jul 03, 2014 at 05:52:13PM -0400, Naoya Horiguchi wrote:
> This patch provides a new system call fincore(2), which provides mincore()-
> like information, i.e. page residency of a given file. But unlike mincore(),
> fincore() has a mode flag which allows us to extract detailed information
> about page cache like pfn and page flag. This kind of information is very
> helpful, for example when applications want to know the file cache status
> to control the IO on their own way.

It's still a nasty multiplexer for multiple different reporting formats
in a single system call.  How about your really just do a fincore that
mirrors mincore instead of piggybacking exports of various internal
flags (tags and page flags onto it.