futex: Restore one of the fast paths eliminated by 38d47c1b7075bd7ec3881141bb3629da58f88dab

This improves futex performance until our user-space code is fixed to use
FUTEX_PRIVATE_FLAG for non-shared futexes.

Signed-off-by: Arve Hjønnevåg <arve@android.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 6a3a5fa1526d..257db2460ab8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -220,6 +220,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 	struct mm_struct *mm = current->mm;
 	struct page *page;
 	int err;
+	struct vm_area_struct *vma;
 
 	/*
 	 * The futex address must be "naturally" aligned.
@@ -245,6 +246,37 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 		return 0;
 	}
 
+	/*
+	 * The futex is hashed differently depending on whether
+	 * it's in a shared or private mapping.  So check vma first.
+	 */
+	vma = find_extend_vma(mm, address);
+	if (unlikely(!vma))
+		return -EFAULT;
+
+	/*
+	 * Permissions.
+	 */
+	if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
+		return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
+
+	/*
+	 * Private mappings are handled in a simple way.
+	 *
+	 * NOTE: When userspace waits on a MAP_SHARED mapping, even if
+	 * it's a read-only handle, it's expected that futexes attach to
+	 * the object not the particular process.  Therefore we use
+	 * VM_MAYSHARE here, not VM_SHARED which is restricted to shared
+	 * mappings of _writable_ handles.
+	 */
+	if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
+		key->both.offset |= FUT_OFF_MMSHARED; /* reference taken on mm */
+		key->private.mm = mm;
+		key->private.address = address;
+		get_futex_key_refs(key);
+		return 0;
+	}
+
 again:
 	err = get_user_pages_fast(address, 1, 1, &page);
 	if (err < 0)