Re: [PATCH 1/2] mm: lock a vma before stack expansion

On Thu, 6 Jul 2023 21:32:10 -0700 Suren Baghdasaryan surenb@google.com wrote:

With recent changes necessitating mmap_lock to be held for write while expanding a stack, per-VMA locks should follow the same rules and be write-locked to prevent page faults into the VMA being expanded. Add the necessary locking.

What are the possible runtime effects of this change?

--- a/mm/mmap.c +++ b/mm/mmap.c @@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address) return -ENOMEM; }

• /* Lock the VMA before expanding to prevent concurrent page faults */
• vma_start_write(vma); /*
  • vma->vm_start/vm_end cannot change under us because the caller
  • is required to hold the mmap_lock in read mode. We need the

@@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address) return -ENOMEM; }

• /* Lock the VMA before expanding to prevent concurrent page faults */
• vma_start_write(vma); /*
  • vma->vm_start/vm_end cannot change under us because the caller
  • is required to hold the mmap_lock in read mode. We need the

-- 2.41.0.255.g8b1d071c50-goog