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Date: Tue, 23 Feb 2016 18:45:05 +0530
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>,
 kirill.shutemov@linux.intel.com, Hugh Dickins <hughd@google.com>
CC: Linux PPC dev <linuxppc-dev@ozlabs.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Subject: Question on follow_page_mask
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Not able to understand the first code block of follow_page_mask
function. follow_huge_addr function is expected to find the page
struct for the given address if it turns out to be a HugeTLB page
but then when it finds the page we bug on if it had been called
with FOLL_GET flag.

	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
	if (!IS_ERR(page)) {
		BUG_ON(flags & FOLL_GET);
		return page;
	}

do_move_page_to_node_array calls follow_page with FOLL_GET which
in turn calls follow_page_mask with FOLL_GET. On POWER, the
function follow_huge_addr is defined and does not return -EINVAL
like the generic one. It returns the page struct if its a HugeTLB
page. Just curious to know what is the purpose behind the BUG_ON.
Thank you.

Regards
Anshuman