Patch "s390/mm: handle PTE-mapped tail pages in fast gup" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

s390/mm: handle PTE-mapped tail pages in fast gup

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
s390-mm-handle-pte-mapped-tail-pages-in-fast-gup.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Gerald Schaefer <[email protected]>
Date: Thu, 17 Mar 2016 15:00:04 +0100
Subject: s390/mm: handle PTE-mapped tail pages in fast gup

From: Gerald Schaefer <[email protected]>

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.

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

@@ -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)++;
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/s390-mm-handle-pte-mapped-tail-pages-in-fast-gup.patch