Re: [PATCH] mm/damon/vaddr: drop redundant mmap_read_lock from damon_va_three_regions

[Cunlong Li <shenxiaogll@gmail.com>]

On Sun, Jun 07, 2026 at 10:36:44AM -0700, SeongJae Park wrote:
> Hello Cunlong,
> 
> On Sun, 07 Jun 2026 21:48:52 +0800 Cunlong Li <shenxiaogll@gmail.com> wrote:
> 
> > __damon_va_three_regions() already walks VMAs under rcu_read_lock(), so
> > the outer mmap_read_lock() is no longer needed.
> 
> The function aims to read a snapshot of the virtual address space.  So I think
> mmap_read_lock() is still required.  Please let me know if I'm missing
> something.
> 
> 
> Thanks,
> SJ
> 
> [...]

Yes — I understand this path is meant to obtain a snapshot of the VMA layout.

My thinking was that an exact snapshot may not be required here, and
the RCU-protected traversal inside __damon_va_three_regions() already
makes the walk safe, so the outer mmap_read_lock() might be redundant.

But, if we want a consistent VMA layout here, mmap_read_lock() is the
right mechanism and rcu_read_lock() alone is not enough.  I'll drop
the patch :)

Thanks,
Cunlong