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From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 05/12] mm: thp: add core routines for thp/pmd migration
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, linux-mm@kvack.org
References: <1478561517-4317-1-git-send-email-n-horiguchi@ah.jp.nec.com>
 <1478561517-4317-6-git-send-email-n-horiguchi@ah.jp.nec.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>,
        Pavel Emelyanov <xemul@parallels.com>, Zi Yan <zi.yan@cs.rutgers.edu>,
        Balbir Singh <bsingharora@gmail.com>, linux-kernel@vger.kernel.org,
        Naoya Horiguchi <nao.horiguchi@gmail.com>
Date: Thu, 10 Nov 2016 13:59:03 +0530
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On 11/08/2016 05:01 AM, Naoya Horiguchi wrote:
> This patch prepares thp migration's core code. These code will be open when
> unmap_and_move() stops unconditionally splitting thp and get_new_page() starts
> to allocate destination thps.
> 

Snip

> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
> ChangeLog v1 -> v2:
> - support pte-mapped thp, doubly-mapped thp
> ---
>  arch/x86/include/asm/pgtable_64.h |   2 +
>  include/linux/swapops.h           |  61 +++++++++++++++
>  mm/huge_memory.c                  | 154 ++++++++++++++++++++++++++++++++++++++
>  mm/migrate.c                      |  44 ++++++++++-
>  mm/pgtable-generic.c              |   3 +-
>  5 files changed, 262 insertions(+), 2 deletions(-)


> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/pgtable-generic.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/pgtable-generic.c
> index 71c5f91..6012343 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/pgtable-generic.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/pgtable-generic.c
> @@ -118,7 +118,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>  {
>  	pmd_t pmd;
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
> +	VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
> +		  !pmd_devmap(*pmdp))

Its a valid VM_BUG_ON check but is it related to THP migration or
just a regular fix up ?