[PATCH] futex: Fix uninterruptible loop due to gate_area

[Hugh Dickins]

commit e6780f7243eddb133cc20ec37fa69317c218b709 upstream.

It was found (by Sasha) that if you use a futex located in the gate
area we get stuck in an uninterruptible infinite loop, much like the
ZERO_PAGE issue.

While looking at this problem, PeterZ realized you'll get into similar
trouble when hitting any install_special_pages() mapping.  And are there
still drivers setting up their own special mmaps without page->mapping,
and without special VM or pte flags to make get_user_pages fail?

In most cases, if page->mapping is NULL, we do not need to retry at all:
Linus points out that even /proc/sys/vm/drop_caches poses no problem,
because it ends up using remove_mapping(), which takes care not to
interfere when the page reference count is raised.

But there is still one case which does need a retry: if memory pressure
called shmem_writepage in between get_user_pages_fast dropping page
table lock and our acquiring page lock, then the page gets switched from
filecache to swapcache (and ->mapping set to NULL) whatever the refcount.
Fault it back in to get the page->mapping needed for key->shared.inode.

Reported-by: Sasha Levin <levinsasha928@xxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[PG: 2.6.34 variable is page, not page_head, since it doesn't have a5b338f2]
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>

diff --git a/kernel/futex.c b/kernel/futex.c
index 98a354d..8b467b4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -264,17 +264,29 @@ again:
 
 	page = compound_head(page);
 	lock_page(page);
+
+	/*
+	 * If page->mapping is NULL, then it cannot be a PageAnon
+	 * page; but it might be the ZERO_PAGE or in the gate area or
+	 * in a special mapping (all cases which we are happy to fail);
+	 * or it may have been a good file page when get_user_pages_fast
+	 * found it, but truncated or holepunched or subjected to
+	 * invalidate_complete_page2 before we got the page lock (also
+	 * cases which we are happy to fail).  And we hold a reference,
+	 * so refcount care in invalidate_complete_page's remove_mapping
+	 * prevents drop_caches from setting mapping to NULL beneath us.
+	 *
+	 * The case we do have to guard against is when memory pressure made
+	 * shmem_writepage move it from filecache to swapcache beneath us:
+	 * an unlikely race, but we do need to retry for page->mapping.
+	 */
 	if (!page->mapping) {
+		int shmem_swizzled = PageSwapCache(page);
 		unlock_page(page);
 		put_page(page);
-		/*
-		* ZERO_PAGE pages don't have a mapping. Avoid a busy loop
-		* trying to find one. RW mapping would have COW'd (and thus
-		* have a mapping) so this page is RO and won't ever change.
-		*/
-		if ((page == ZERO_PAGE(address)))
-			return -EFAULT;
-		goto again;
+		if (shmem_swizzled)
+			goto again;
+		return -EFAULT;
 	}
 
 	/*
-- 
1.7.9.6

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