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Date: Sun, 29 Mar 2015 20:51:38 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCHv4 15/24] mm, thp: remove infrastructure for handling
 splitting PMDs
Message-ID: <20150329175138.GC976@node.dhcp.inet.fi>
References: <1425486792-93161-1-git-send-email-kirill.shutemov@linux.intel.com>
 <1425486792-93161-16-git-send-email-kirill.shutemov@linux.intel.com>
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To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Dave Hansen <dave.hansen@intel.com>, Hugh Dickins <hughd@google.com>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>, Christoph Lameter <cl@gentwo.org>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, Steve Capper <steve.capper@linaro.org>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>, Jerome Marchand <jmarchan@redhat.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org

On Sun, Mar 29, 2015 at 09:40:07PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:
> 
> > With new refcounting we don't need to mark PMDs splitting. Let's drop code
> > to handle this.
> >
> > Arch-specific code will removed separately.
> >
> 
> Can you explain this more ? Why we don't care of PMD splitting case even
> w.r.t to split_huge_page() ? 

It used to be required to keep kernel from updating page refcounts while
we splitting the page. Now with split_huge_pmd() we can split one PMD a
time without blocking refcounting. Once all PMDs split we can freeze the
page's refcounts with compound lock[1] and split underlying compound page.

[1] compound lock is replaced with migration entries by the end of the
    patchset.

-- 
 Kirill A. Shutemov

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