linux-kernel - Re: [PATCHv4 08/25] thp: support file pages in zap_huge

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Date:	Mon, 21 Mar 2016 17:39:00 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Matthew Wilcox <willy@...ux.intel.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>,
	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
Subject: Re: [PATCHv4 08/25] thp: support file pages in zap_huge_pmd()

On Mon, Mar 21, 2016 at 10:03:29AM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill@...temov.name> writes:
> 
> > [ text/plain ]
> > On Fri, Mar 18, 2016 at 07:23:41PM +0530, Aneesh Kumar K.V wrote:
> >> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> writes:
> >> 
> >> > [ text/plain ]
> >> > split_huge_pmd() for file mappings (and DAX too) is implemented by just
> >> > clearing pmd entry as we can re-fill this area from page cache on pte
> >> > level later.
> >> >
> >> > This means we don't need deposit page tables when file THP is mapped.
> >> > Therefore we shouldn't try to withdraw a page table on zap_huge_pmd()
> >> > file THP PMD.
> >> 
> >> Archs like ppc64 use deposited page table to track the hardware page
> >> table slot information. We probably may want to add hooks which arch can
> >> use to achieve the same even with file THP 
> >
> > Could you describe more on what kind of information you're talking about?
> >
> 
> Hardware page table in ppc64 requires us to map each subpage of the huge
> page. This is needed because at low level we use segment base page size
> to find the hash slot and on TLB miss, we use the faulting address and
> base page size (which is 64k even with THP) to find whether we have
> the page mapped in hash page table. Since we use base page size of 64K,
> we need to make sure that subpages are mapped (on demand) in hash page
> table. If we have then mapped we also need to track their hash table
> slot information so that we can clear it on invalidate of hugepage.
> 
> With THP we used the deposited page table to store the hash slot
> information.

Could you prepare the patch with these hooks so we can discuss it details?
I still have hard time wrap my had around this.

I think you have the same problem with DAX huge pages. Or you don't care
about DAX?

-- 
 Kirill A. Shutemov