[patch] futex: Fix v4.6+ ltp futex_wait04 regression

[Mike Galbraith <umgwanakikbuti@gmail.com>]

Hi Mel (who is out of the office today),

I initially reported this on the rt-users list, thinking at the time
that it was only showing up in -rt kernels, but that turned out to not
be the case, it just requires an enterprise config for some reason mm
folks will likely recognize instantly.  I just happen to use same when
building devel -rt trees to at least get some build warning coverage.

Below is a stab at fixing the thing up while you're off doing whatever
an Irishman does on a national holiday (hm;).  If it's ok as is, fine,
I saved you a couple minutes.  If not, oh well, consider it diagnosis.

65d8fc77 futex: Remove requirement for lock_page() in get_futex_key()
introduced a regression of futex_wait04 when an enterprise size config is
used.  Per trace_printk(), when we assign page = compound_head(page), we're
subsequently using a different page than the one we locked into memory,
thus mucking up references.

Fixes: 65d8fc77 futex: Remove requirement for lock_page() in get_futex_key()
Signed-off-by: Mike Galbraith <umgwanakikbuit@gmail.com>
---
 kernel/futex.c |   26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -469,7 +469,7 @@ get_futex_key(u32 __user *uaddr, int fsh
 {
 	unsigned long address = (unsigned long)uaddr;
 	struct mm_struct *mm = current->mm;
-	struct page *page;
+	struct page *page, *pinned = NULL;
 	struct address_space *mapping;
 	int err, ro = 0;
 
@@ -530,8 +530,18 @@ get_futex_key(u32 __user *uaddr, int fsh
 	 * considered here and page lock forces unnecessarily serialization
 	 * From this point on, mapping will be re-verified if necessary and
 	 * page lock will be acquired only if it is unavoidable
-	 */
+	 *
+	 * If we're dealing with a compound page, save our reference to the
+	 * page we locked in memory above, and take a new reference on the
+	 * page head, dropping the previously pinned page reference on retry.
+	 */
+	if (unlikely(pinned && page != pinned))
+		put_page(pinned);
+	pinned = page;
 	page = compound_head(page);
+	if (unlikely(pinned != page))
+		get_page(page);
+
 	mapping = READ_ONCE(page->mapping);
 
 	/*
@@ -560,12 +570,14 @@ get_futex_key(u32 __user *uaddr, int fsh
 		lock_page(page);
 		shmem_swizzled = PageSwapCache(page) || page->mapping;
 		unlock_page(page);
-		put_page(page);
 
-		if (shmem_swizzled)
+		if (shmem_swizzled) {
+			put_page(page);
 			goto again;
+		}
 
-		return -EFAULT;
+		err = -EFAULT;
+		goto out;
 	}
 
 	/*
@@ -654,12 +666,14 @@ get_futex_key(u32 __user *uaddr, int fsh
 
 		key->both.offset |= FUT_OFF_INODE; /* inode-based key */
 		key->shared.inode = inode;
-		key->shared.pgoff = basepage_index(page);
+		key->shared.pgoff = basepage_index(pinned);
 		rcu_read_unlock();
 	}
 
 out:
 	put_page(page);
+	if (unlikely(pinned != page))
+		put_page(pinned);
 	return err;
 }