From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>, Vlastimil Babka <vbabka@suse.cz>,
        Christoph Lameter <cl@gentwo.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Jerome Marchand <jmarchan@redhat.com>, Yang Shi <yang.shi@linaro.org>,
        Sasha Levin <sasha.levin@oracle.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCHv4 04/25] rmap: support file thp
In-Reply-To: <1457737157-38573-5-git-send-email-kirill.shutemov@linux.intel.com>
References: <1457737157-38573-1-git-send-email-kirill.shutemov@linux.intel.com> <1457737157-38573-5-git-send-email-kirill.shutemov@linux.intel.com>
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Date: Fri, 18 Mar 2016 15:10:06 +0530
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"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:

> [ text/plain ]
> Naive approach: on mapping/unmapping the page as compound we update
> ->_mapcount on each 4k page. That's not efficient, but it's not obvious
> how we can optimize this. We can look into optimization later.
>
> PG_double_map optimization doesn't work for file pages since lifecycle
> of file pages is different comparing to anon pages: file page can be
> mapped again at any time.
>

Can you explain this more ?. We added PG_double_map so that we can keep
page_remove_rmap simpler. So if it isn't a compound page we still can do

	if (!atomic_add_negative(-1, &page->_mapcount))

I am trying to understand why we can't use that with file pages ?

-aneesh