netdev - Re: [RFC] Make find_tcp

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Message-ID: <CAJuCfpHxpZVnpM2bE25MeFK7CrSsO_pGaYuwVNzre47bb1Jh_w@mail.gmail.com>
Date: Fri, 3 May 2024 15:29:16 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org, netdev@...r.kernel.org, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Arjun Roy <arjunroy@...gle.com>
Subject: Re: [RFC] Make find_tcp_vma() more efficient

On Fri, Apr 26, 2024 at 10:15 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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)

I went through all uses of vma_end_read() to convince myself this is
safe with CONFIG_PER_VMA_LOCK=n and the change seems fine from
correctness POV. However the fact that in this configuration
lock_vma_under_mmap_lock()==NOP and vma_end_read()==mmap_read_unlock()
does not feel right to me. Current code is more explicit about which
lock is held and I think it's easier to understand.
A new interface like below might be a bit better but I'm not sure if
it's worth it:

#ifdef CONFIG_PER_VMA_LOCK
static inline void mmap_to_vma_lock(struct vm_area_struct *vma)
{
       down_read(&vma->vm_lock->lock);
       mmap_read_unlock(vma->vm_mm);
}

static inline void mmap_or_vma_unlock(struct vm_area_struct *vma)
{
       vma_end_read(vma);
}

#else /* CONFIG_PER_VMA_LOCK */

static inline void mmap_to_vma_lock(struct vm_area_struct *vma) {}
static inline void mmap_or_vma_unlock(struct vm_area_struct *vma)
{
       mmap_read_unlock(vma->vm_mm);
}
#endif /* CONFIG_PER_VMA_LOCK */

>
> 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);