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

[Lorenzo Stoakes]

On Wed, Apr 29, 2026 at 11:20:02AM -0700, Dave Hansen wrote:
>
> 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>

Yeah, LGTM again, though am not a networking guy so:

Acked-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>

> 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);
> _