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

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Message-ID: <20160218121909.GA28184@node.shutemov.name>
Date:	Thu, 18 Feb 2016 14:19:09 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	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 Tue, Feb 16, 2016 at 07:31:58AM -0800, Dave Hansen wrote:
> On 02/16/2016 02:00 AM, Kirill A. Shutemov wrote:
> > 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.
> 
> Ahh...  Do we just never split in any fault contexts, or do we just
> retry the fault?

In fault contexts we would just continue fault handling as if we had
pmd_none().

-- 
 Kirill A. Shutemov