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

[Suren Baghdasaryan]

On Fri, May 8, 2026 at 10:32 AM Lorenzo Stoakes <ljs@xxxxxxxxxx> wrote:
>
> 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().

Indeed!

> >
> > Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> Yeah, LGTM again, though am not a networking guy so:
>
> Acked-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@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);
> > _