path: root/kernel
diff options
authorHugh Dickins <>2011-12-31 11:44:01 -0800
committerGreg Kroah-Hartman <>2012-01-06 14:17:22 -0800
commit04a763aed3c611460ef4888d14a1f5101e8373bc (patch)
treee80554a07a15405f543339d22be625c3b7edd3a6 /kernel
parent6a8f198e5d3573276fba8efa3d2e006da5f02e1f (diff)
futex: Fix uninterruptible loop due to gate_area
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 <> Signed-off-by: Hugh Dickins <> Signed-off-by: Linus Torvalds <> Signed-off-by: Greg Kroah-Hartman <>
Diffstat (limited to 'kernel')
1 files changed, 20 insertions, 8 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 11cbe052b2e8..e6160fa842e4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -314,17 +314,29 @@ again:
+ /*
+ * If page_head->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_head->mapping.
+ */
if (!page_head->mapping) {
+ int shmem_swizzled = PageSwapCache(page_head);
- /*
- * 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_head == ZERO_PAGE(address)))
- return -EFAULT;
- goto again;
+ if (shmem_swizzled)
+ goto again;
+ return -EFAULT;