linux-kernel - [PATCH] mm, hugetlb, soft_offline: save compound page order before page migration

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Date:   Tue, 12 Sep 2017 22:43:06 +0200
From:   Alexandru Moise <00moses.alexander00@...il.com>
To:     linux-kernel@...r.kernel.org, khandual@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, mhocko@...e.com, aarcange@...hat.com,
        minchan@...nel.org, hillf.zj@...baba-inc.com, shli@...com,
        rppt@...ux.vnet.ibm.com, kirill.shutemov@...ux.intel.com,
        mgorman@...hsingularity.net, rientjes@...gle.com, riel@...hat.com,
        linux-mm@...ck.org
Subject: [PATCH] mm, hugetlb, soft_offline: save compound page order before
 page migration

This fixes a bug in madvise() where if you'd try to soft offline a
hugepage via madvise(), while walking the address range you'd end up,
using the wrong page offset due to attempting to get the compound
order of a former but presently not compound page, due to dissolving
the huge page (since c3114a8).

Signed-off-by: Alexandru Moise <00moses.alexander00@...il.com>
---
 mm/madvise.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 21261ff0466f..25bade36e9ca 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -625,18 +625,26 @@ static int madvise_inject_error(int behavior,
 {
 	struct page *page;
 	struct zone *zone;
+	unsigned int order;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	for (; start < end; start += PAGE_SIZE <<
-				compound_order(compound_head(page))) {
+
+	for (; start < end; start += PAGE_SIZE << order) {
 		int ret;
 
 		ret = get_user_pages_fast(start, 1, 0, &page);
 		if (ret != 1)
 			return ret;
 
+		/*
+		 * When soft offlining hugepages, after migrating the page
+		 * we dissolve it, therefore in the second loop "page" will
+		 * no longer be a compound page, and order will be 0.
+		 */
+		order = compound_order(compound_head(page));
+
 		if (PageHWPoison(page)) {
 			put_page(page);
 			continue;
-- 
2.14.1