linux-kernel - Re: [PATCHv2 13/28] thp: support file pages in zap_huge

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Message-ID: <20160216100023.GC46557@black.fi.intel.com>
Date:	Tue, 16 Feb 2016 13:00:23 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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
Subject: Re: [PATCHv2 13/28] thp: support file pages in zap_huge_pmd()

On Fri, Feb 12, 2016 at 10:33:37AM -0800, Dave Hansen wrote:
> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote:
> > For file pages we don't deposit page table on mapping: no need to
> > withdraw it.
> 
> I thought the deposit thing was to guarantee we could always do a PMD
> split.  It still seems like if you wanted to split a huge-tmpfs page,
> you'd need to first split the PMD which might need the deposited one.
> 
> Why not?

For file thp, split_huge_pmd() is implemented by clearing out the pmd: we
can setup and fill pte table later. Therefore no need to deposit page
table -- we would not use it. DAX does the same.

-- 
 Kirill A. Shutemov