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

[Alexandru Moise]

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.alexande...@gmail.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