Re: [PATCH v4 3/3] fs/proc/task_mmu: execute PROCMAP_QUERY ioctl under per-vma locks

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Aug 08, 2025 at 08:28:49AM -0700, Suren Baghdasaryan wrote:
> Utilize per-vma locks to stabilize vma after lookup without taking
> mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is
> contended, we fall back to mmap_lock but take it only momentarily
> to lock the vma and release the mmap_lock. In a very unlikely case
> of vm_refcnt overflow, this fall back path will fail and ioctl is
> done under mmap_lock protection.
>
> This change is designed to reduce mmap_lock contention and prevent
> PROCMAP_QUERY ioctl calls from blocking address space updates.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Acked-by: SeongJae Park <sj@kernel.org>

All LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  fs/proc/task_mmu.c | 103 +++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 85 insertions(+), 18 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c0968d293b61..e64cf40ce9c4 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -132,6 +132,12 @@ static void release_task_mempolicy(struct proc_maps_private *priv)
>
>  #ifdef CONFIG_PER_VMA_LOCK
>
> +static void reset_lock_ctx(struct proc_maps_locking_ctx *lock_ctx)
> +{
> +	lock_ctx->locked_vma = NULL;
> +	lock_ctx->mmap_locked = false;
> +}

Thanks!

> +
>  static void unlock_ctx_vma(struct proc_maps_locking_ctx *lock_ctx)
>  {
>  	if (lock_ctx->locked_vma) {
> @@ -157,8 +163,7 @@ static inline bool lock_vma_range(struct seq_file *m,
>  		lock_ctx->mmap_locked = true;
>  	} else {
>  		rcu_read_lock();
> -		lock_ctx->locked_vma = NULL;
> -		lock_ctx->mmap_locked = false;
> +		reset_lock_ctx(lock_ctx);
>  	}
>
>  	return true;
> @@ -522,28 +527,90 @@ static int pid_maps_open(struct inode *inode, struct file *file)
>  		PROCMAP_QUERY_VMA_FLAGS				\
>  )
>
> -static int query_vma_setup(struct mm_struct *mm)
> +#ifdef CONFIG_PER_VMA_LOCK
> +
> +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	return mmap_read_lock_killable(mm);
> +	reset_lock_ctx(lock_ctx);
> +
> +	return 0;
>  }
>
> -static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma)
> +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
>  {
> -	mmap_read_unlock(mm);
> +	if (lock_ctx->mmap_locked) {
> +		mmap_read_unlock(lock_ctx->mm);
> +		lock_ctx->mmap_locked = false;
> +	} else {
> +		unlock_ctx_vma(lock_ctx);
> +	}
> +}
> +
> +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> +						     unsigned long addr)
> +{
> +	struct mm_struct *mm = lock_ctx->mm;
> +	struct vm_area_struct *vma;
> +	struct vma_iterator vmi;
> +
> +	if (lock_ctx->mmap_locked)
> +		return find_vma(mm, addr);
> +
> +	/* Unlock previously locked VMA and find the next one under RCU */
> +	unlock_ctx_vma(lock_ctx);
> +	rcu_read_lock();
> +	vma_iter_init(&vmi, mm, addr);
> +	vma = lock_next_vma(mm, &vmi, addr);
> +	rcu_read_unlock();
> +
> +	if (!vma)
> +		return NULL;
> +
> +	if (!IS_ERR(vma)) {
> +		lock_ctx->locked_vma = vma;
> +		return vma;
> +	}
> +
> +	if (PTR_ERR(vma) == -EAGAIN) {
> +		/* Fallback to mmap_lock on vma->vm_refcnt overflow */
> +		mmap_read_lock(mm);
> +		vma = find_vma(mm, addr);
> +		lock_ctx->mmap_locked = true;
> +	}

Thanks this is great!

> +
> +	return vma;
>  }
>
> -static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr)
> +#else /* CONFIG_PER_VMA_LOCK */
> +
> +static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx)
> +{
> +	return mmap_read_lock_killable(lock_ctx->mm);
> +}
> +
> +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> +{
> +	mmap_read_unlock(lock_ctx->mm);
> +}
> +
> +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,
> +						     unsigned long addr)
>  {
> -	return find_vma(mm, addr);
> +	return find_vma(lock_ctx->mm, addr);
>  }
>
> -static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
> +#endif  /* CONFIG_PER_VMA_LOCK */
> +
> +static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx,
>  						 unsigned long addr, u32 flags)
>  {
>  	struct vm_area_struct *vma;
>
>  next_vma:
> -	vma = query_vma_find_by_addr(mm, addr);
> +	vma = query_vma_find_by_addr(lock_ctx, addr);
> +	if (IS_ERR(vma))
> +		return vma;
> +
>  	if (!vma)
>  		goto no_vma;
>
> @@ -584,11 +651,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm,
>  	return ERR_PTR(-ENOENT);
>  }
>
> -static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
> +static int do_procmap_query(struct mm_struct *mm, void __user *uarg)
>  {
> +	struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
>  	struct procmap_query karg;
>  	struct vm_area_struct *vma;
> -	struct mm_struct *mm;
>  	const char *name = NULL;
>  	char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
>  	__u64 usize;
> @@ -615,17 +682,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	if (!!karg.build_id_size != !!karg.build_id_addr)
>  		return -EINVAL;
>
> -	mm = priv->lock_ctx.mm;
>  	if (!mm || !mmget_not_zero(mm))
>  		return -ESRCH;
>
> -	err = query_vma_setup(mm);
> +	err = query_vma_setup(&lock_ctx);
>  	if (err) {
>  		mmput(mm);
>  		return err;
>  	}
>
> -	vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);
> +	vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags);
>  	if (IS_ERR(vma)) {
>  		err = PTR_ERR(vma);
>  		vma = NULL;
> @@ -710,7 +776,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	}
>
>  	/* unlock vma or mmap_lock, and put mm_struct before copying data to user */
> -	query_vma_teardown(mm, vma);
> +	query_vma_teardown(&lock_ctx);
>  	mmput(mm);
>
>  	if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
> @@ -730,7 +796,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg)
>  	return 0;
>
>  out:
> -	query_vma_teardown(mm, vma);
> +	query_vma_teardown(&lock_ctx);
>  	mmput(mm);
>  	kfree(name_buf);
>  	return err;
> @@ -743,7 +809,8 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l
>
>  	switch (cmd) {
>  	case PROCMAP_QUERY:
> -		return do_procmap_query(priv, (void __user *)arg);
> +		/* priv->lock_ctx.mm is set during file open operation */

Thanks!

> +		return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
>  	default:
>  		return -ENOIOCTLCMD;
>  	}
> --
> 2.50.1.703.g449372360f-goog
>