Re: [PATCH v2 3/3] fs/proc/task_mmu: execute PROCMAP_QUERY ioctl under per-vma locks

On Tue, Aug 5, 2025 at 7:18 AM Vlastimil Babka vbabka@suse.cz wrote:

On 8/5/25 1:15 AM, 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

fs/proc/task_mmu.c | 81 +++++++++++++++++++++++++++++++++++++--------- 1 file changed, 65 insertions(+), 16 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 843577aa7a32..1d06ecdbef6f 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -517,28 +517,78 @@ 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);
  

• lock_ctx->locked_vma = NULL;
  

• lock_ctx->mmap_locked = false;
  

• 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);
  

• else
  

•         unlock_vma(lock_ctx);
  

}

-static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr) +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);
  

• struct vm_area_struct *vma;
  

• struct vma_iterator vmi;
  
  

Hm I think we can reach here with lock_ctx->mmap_locked being true via "goto next_vma" in query_matching_vma(). In that case we should just "return find_vma()" and doing the below is wrong, no?

Ah, you are quite right. I should handle mmap_locked differently in query_vma_find_by_addr(). I will post the fix shortly.

...

• unlock_vma(lock_ctx);
  

• rcu_read_lock();
  

• vma_iter_init(&vmi, lock_ctx->mm, addr);
  

• vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
  

• rcu_read_unlock();
  

• if (!IS_ERR_OR_NULL(vma)) {
  

•         lock_ctx->locked_vma = vma;
  

• } else if (PTR_ERR(vma) == -EAGAIN) {
  

•         /* Fallback to mmap_lock on vma->vm_refcnt overflow */
  

•         mmap_read_lock(lock_ctx->mm);
  

•         vma = find_vma(lock_ctx->mm, addr);
  

•         lock_ctx->mmap_locked = true;
  

• }
  

• return vma;
  

}