linux-kernel - Re: [PATCHv4 04/25] rmap: support file thp

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Message-ID: <87d1qs9lah.fsf@linux.vnet.ibm.com>
Date:	Fri, 18 Mar 2016 15:10:06 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Yang Shi <yang.shi@...aro.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv4 04/25] rmap: support file thp

"Kirill A. Shutemov" <kirill.shutemov@...ux.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