[PATCH v4 5/5] tcp: Remove mmap_lock fallback path

[Suren Baghdasaryan <surenb@google.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

Previously, the per-VMA locking could fail in the face of writers
which necessitates a fallback to mmap_lock. The new
lock_vma_under_rcu_wait() will wait for writers instead of failing.

Use the new helper. Wait for writers. Remove the fallback to mmap_lock.
The fallback removal does not affect NOMMU case because TCP_ZEROCOPY
is gated on CONFIG_MMU.

This really is a nice cleanup. It removes the need to pass the lock
state back and forth to find_tcp_vma().

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Tested-by: syzbot@syzkaller.appspotmail.com
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: linux-mm@kvack.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: netdev@vger.kernel.org
---
 net/ipv4/tcp.c | 31 +++++++++----------------------
 1 file changed, 9 insertions(+), 22 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 455441f1b694..62defe70f3ce 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2167,27 +2167,18 @@ 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);
+	struct vm_area_struct *vma = vma_start_read_unlocked(mm, address);
 
-	if (vma) {
-		if (vma->vm_ops != &tcp_vm_ops) {
-			vma_end_read(vma);
-			return NULL;
-		}
-		*mmap_locked = false;
-		return vma;
-	}
+	if (!vma)
+		return NULL;
 
-	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;
 }
 
@@ -2208,7 +2199,6 @@ 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;
@@ -2233,7 +2223,7 @@ 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;
 
@@ -2315,10 +2305,7 @@ 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);
-- 
2.55.0.654.g21b8a5bc05-goog