[PATCH 5/6] tcp: Remove mmap_lock fallback path

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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.

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@xxxxxxxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
---

 b/net/ipv4/tcp.c |   31 +++++++++----------------------
 1 file changed, 9 insertions(+), 22 deletions(-)

diff -puN net/ipv4/tcp.c~ipv4-tcp-vma-waiter net/ipv4/tcp.c
--- a/net/ipv4/tcp.c~ipv4-tcp-vma-waiter	2026-04-29 11:18:51.870676498 -0700
+++ b/net/ipv4/tcp.c	2026-04-29 11:18:51.874676652 -0700
@@ -2171,27 +2171,18 @@ static void tcp_zc_finalize_rx_tstamp(st
 }
 
 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 = lock_vma_under_rcu_wait(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;
 }
 
@@ -2212,7 +2203,6 @@ static int tcp_zerocopy_receive(struct s
 	u32 seq = tp->copied_seq;
 	u32 total_bytes_to_map;
 	int inq = tcp_inq(sk);
-	bool mmap_locked;
 	int ret;
 
 	zc->copybuf_len = 0;
@@ -2237,7 +2227,7 @@ static int tcp_zerocopy_receive(struct s
 		return 0;
 	}
 
-	vma = find_tcp_vma(current->mm, address, &mmap_locked);
+	vma = find_tcp_vma(current->mm, address);
 	if (!vma)
 		return -EINVAL;
 
@@ -2319,10 +2309,7 @@ static int tcp_zerocopy_receive(struct s
 						   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);
_