Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding spinlocks

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 22 Dec 2021 20:48:28 +0100 Manfred Spraul <manfred@colorfullife.com> wrote:

> One codepath in find_alloc_undo() calls kvfree() while holding a spinlock.
> Since vfree() can sleep this is a bug.
> 
> Previously, the code path used kfree(), and kfree() is safe to be called
> while holding a spinlock.
> 
> Minghao proposed to fix this by updating find_alloc_undo().
> 
> Alternate proposal to fix this: Instead of changing find_alloc_undo(),
> change kvfree() so that the same rules as for kfree() apply:
> Having different rules for kfree() and kvfree() just asks for bugs.
> 
> Disadvantage: Releasing vmalloc'ed memory will be delayed a bit.

I know we've been around this loop a bunch of times and deferring was
considered.   But I forget the conclusion.  IIRC, mhocko was involved?

> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -610,12 +610,12 @@ EXPORT_SYMBOL(kvmalloc_node);
>   * It is slightly more efficient to use kfree() or vfree() if you are certain
>   * that you know which one to use.
>   *
> - * Context: Either preemptible task context or not-NMI interrupt.
> + * Context: Any context except NMI interrupt.
>   */
>  void kvfree(const void *addr)
>  {
>  	if (is_vmalloc_addr(addr))
> -		vfree(addr);
> +		vfree_atomic(addr);
>  	else
>  		kfree(addr);
>  }