On Fri, Apr 26, 2024 at 10:15 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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);
