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@xxxxxxxxxx>
> Acked-by: SeongJae Park <sj@xxxxxxxxxx>
All LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
> 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
>
