[PATCH 4.5 015/124] s390/mm: handle PTE-mapped tail pages in fast gup

[Greg Kroah-Hartman]

4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>

commit fc897c95e91451271cd707ee0f71022b9b201ce9 upstream.

With the THP refcounting rework it is possible to see THP compound tail
pages mapped with PTEs during a THP split. This needs to be considered
when using page_cache_get_speculative(), which will always fail on tail
pages because ->_count is always zero. commit 7aef4172 "mm: handle
PTE-mapped tail pages in gerneric fast gup implementaiton" fixed it for
the generic fast gup code by using compound_head(page) instead of page,
but not for s390.

This patch is a 1:1 adaption of commit 7aef4172 for the s390 fast gup
code. Without this fix, gup will fall back to the slow path or fail
in the unlikely scenario that we hit a THP under splitting in-between
the page table split and the compound page split.

Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/s390/mm/gup.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/arch/s390/mm/gup.c
+++ b/arch/s390/mm/gup.c
@@ -20,9 +20,9 @@
 static inline int gup_pte_range(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
 		unsigned long end, int write, struct page **pages, int *nr)
 {
+	struct page *head, *page;
 	unsigned long mask;
 	pte_t *ptep, pte;
-	struct page *page;
 
 	mask = (write ? _PAGE_PROTECT : 0) | _PAGE_INVALID | _PAGE_SPECIAL;
 
@@ -37,12 +37,14 @@ static inline int gup_pte_range(pmd_t *p
 			return 0;
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
-		if (!page_cache_get_speculative(page))
+		head = compound_head(page);
+		if (!page_cache_get_speculative(head))
 			return 0;
 		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
-			put_page(page);
+			put_page(head);
 			return 0;
 		}
+		VM_BUG_ON_PAGE(compound_head(page) != head, page);
 		pages[*nr] = page;
 		(*nr)++;