mm/hugetlb: improve locking in dissolve_free_huge_pages()

For every pfn aligned to minimum_order, dissolve_free_huge_pages() will
call dissolve_free_huge_page() which takes the hugetlb spinlock, even if
the page is not huge at all or a hugepage that is in-use.

Improve this by doing the PageHuge() and page_count() checks already in
dissolve_free_huge_pages() before calling dissolve_free_huge_page().  In
dissolve_free_huge_page(), when holding the spinlock, those checks need
to be revalidated.

Link: http://lkml.kernel.org/r/20160926172811.94033-4-gerald.schaefer@de.ibm.com
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Rui Teng <rui.teng@linux.vnet.ibm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 91ae1f567997..770d83eb3f48 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1476,14 +1476,20 @@ out:
 int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
 {
         unsigned long pfn;
+        struct page *page;
         int rc = 0;
 
         if (!hugepages_supported())
                 return rc;
 
-        for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order)
-                if (rc = dissolve_free_huge_page(pfn_to_page(pfn)))
-                        break;
+        for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order) {
+                page = pfn_to_page(pfn);
+                if (PageHuge(page) && !page_count(page)) {
+                        rc = dissolve_free_huge_page(page);
+                        if (rc)
+                                break;
+                }
+        }
 
         return rc;
 }