[RFC] Make find_tcp_vma() more efficient

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

Liam asked me if we could do away with the "bool *mmap_locked"
parameter, and the problem is that some architctures don't support
CONFIG_PER_VMA_LOCK yet.  But we can abstract it ... something like this
maybe?

(not particularly proposing this for inclusion; just wrote it and want
to get it out of my tree so I can get back to other projects.  If anyone
wants it, they can test it and submit it for inclusion and stick my
S-o-B on it)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9849dfda44d4..570763351508 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -779,11 +779,22 @@ static inline void assert_fault_locked(struct vm_fault *vmf)
 struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 					  unsigned long address);
 
+static inline void lock_vma_under_mmap_lock(struct vm_area_struct *vma)
+{
+	down_read(&vma->vm_lock->lock);
+	mmap_read_unlock(vma->vm_mm);
+}
+
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline bool vma_start_read(struct vm_area_struct *vma)
 		{ return false; }
-static inline void vma_end_read(struct vm_area_struct *vma) {}
+static inline void vma_end_read(struct vm_area_struct *vma)
+{
+	mmap_read_unlock(vma->vm_mm);
+}
+
+static inline void lock_vma_under_mmap_lock(struct vm_area_struct *vma) {}
 static inline void vma_start_write(struct vm_area_struct *vma) {}
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 		{ mmap_assert_write_locked(vma->vm_mm); }
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f23b97777ea5..e763916e5185 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2051,27 +2051,25 @@ static void tcp_zc_finalize_rx_tstamp(struct sock *sk,
 }
 
 static struct vm_area_struct *find_tcp_vma(struct mm_struct *mm,
-					   unsigned long address,
-					   bool *mmap_locked)
+					   unsigned long address)
 {
 	struct vm_area_struct *vma = lock_vma_under_rcu(mm, address);
 
-	if (vma) {
-		if (vma->vm_ops != &tcp_vm_ops) {
-			vma_end_read(vma);
+	if (!vma) {
+		mmap_read_lock(mm);
+		vma = vma_lookup(mm, address);
+		if (vma) {
+			lock_vma_under_mmap_lock(vma);
+		} else {
+			mmap_read_unlock(mm);
 			return NULL;
 		}
-		*mmap_locked = false;
-		return vma;
 	}
-
-	mmap_read_lock(mm);
-	vma = vma_lookup(mm, address);
-	if (!vma || vma->vm_ops != &tcp_vm_ops) {
-		mmap_read_unlock(mm);
+	if (vma->vm_ops != &tcp_vm_ops) {
+		vma_end_read(vma);
 		return NULL;
 	}
-	*mmap_locked = true;
+
 	return vma;
 }
 
@@ -2092,7 +2090,6 @@ static int tcp_zerocopy_receive(struct sock *sk,
 	u32 seq = tp->copied_seq;
 	u32 total_bytes_to_map;
 	int inq = tcp_inq(sk);
-	bool mmap_locked;
 	int ret;
 
 	zc->copybuf_len = 0;
@@ -2117,7 +2114,7 @@ static int tcp_zerocopy_receive(struct sock *sk,
 		return 0;
 	}
 
-	vma = find_tcp_vma(current->mm, address, &mmap_locked);
+	vma = find_tcp_vma(current->mm, address);
 	if (!vma)
 		return -EINVAL;
 
@@ -2194,10 +2191,7 @@ static int tcp_zerocopy_receive(struct sock *sk,
 						   zc, total_bytes_to_map);
 	}
 out:
-	if (mmap_locked)
-		mmap_read_unlock(current->mm);
-	else
-		vma_end_read(vma);
+	vma_end_read(vma);
 	/* Try to copy straggler data. */
 	if (!ret)
 		copylen = tcp_zc_handle_leftover(zc, sk, skb, &seq, copybuf_len, tss);