Re: [PATCH 1/3] mm: only assert mmap locks if CONFIG_DEBUG_VM or lockdep is set

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 11/10/26 17:57, Lorenzo Stoakes (ARM) wrote:
> For a long while we have asserted the mmap lock being held regardless of
> whether or not the kernel is a debug build or not.
> 
> In general, given that these have been added as programmatic asserts that
> nobody did something stupid, production kernels are simply wasting their
> time checking for this.
> 
> It also means there is a two tier system - where some programmatic
> asserts (e.g. VM_WARN_ON_ONCE()-checked conditions) are only checked for if
> CONFIG_DEBUG_VM is enabled, whereas others are checked unconditionally.
> 
> If we genuinely want to unconditionally check for the mmap lock being held
> at runtime, then that should be made clear in the code doing it.
> 
> Regardless, if CONFIG_LOCKDEP is enabled, ensure that the lockdep
> assertions are performed.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  include/linux/mmap_lock.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 9c3a9c6525c1..9367430ee53c 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -75,12 +75,14 @@ static inline void __mmap_lock_trace_released(struct mm_struct *mm, bool write)
>  
>  static inline void mmap_assert_locked(const struct mm_struct *mm)
>  {
> -	rwsem_assert_held(&mm->mmap_lock);
> +	if (IS_ENABLED(CONFIG_DEBUG_VM) || IS_ENABLED(CONFIG_LOCKDEP))
> +		rwsem_assert_held(&mm->mmap_lock);
>  }
>  
>  static inline void mmap_assert_write_locked(const struct mm_struct *mm)
>  {
> -	rwsem_assert_held_write(&mm->mmap_lock);
> +	if (IS_ENABLED(CONFIG_DEBUG_VM) || IS_ENABLED(CONFIG_LOCKDEP))
> +		rwsem_assert_held_write(&mm->mmap_lock);
>  }
>  
>  #ifdef CONFIG_LOCKDEP
>