Re: [PATCH v2 2/4] userfaultfd: remove (VM_)BUG_ON()s

[David Hildenbrand]

On 07.06.25 08:40, Tal Zussman wrote:
> BUG_ON() is deprecated [1]. Convert all the BUG_ON()s and VM_BUG_ON()s
> to use VM_WARN_ON_ONCE().
>
> While at it, also convert the WARN_ON_ONCE()s in move_pages() to use
> VM_WARN_ON_ONCE(), as the relevant conditions are already checked in
> validate_range() in move_pages()'s caller.
>
> [1] https://www.kernel.org/doc/html/v6.15/process/coding-style.html#use-warn-rather-than-bug
>
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---


[...]

>   	if (ctx->features & UFFD_FEATURE_SIGBUS)
>   		goto out;
> @@ -411,12 +411,11 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>   		 * to be sure not to return SIGBUS erroneously on
>   		 * nowait invocations.
>   		 */
> -		BUG_ON(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
> +		VM_WARN_ON_ONCE(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
>   #ifdef CONFIG_DEBUG_VM
>   		if (printk_ratelimit()) {
> -			printk(KERN_WARNING
> -			       "FAULT_FLAG_ALLOW_RETRY missing %x\n",
> -			       vmf->flags);
> +			pr_warn("FAULT_FLAG_ALLOW_RETRY missing %x\n",
> +				vmf->flags);

You didn't cover that in the patch description.

I do wonder if we really still want the dump_stack() here and could 
simplify to

	pr_warn_ratelimited().

But I recall that the stack was helpful at least once for me (well, I 
was able to reproduce and could have figured it out differently.).

[...]

>   	err = uffd_move_lock(mm, dst_start, src_start, &dst_vma, &src_vma);
>   	if (err)
> @@ -1867,9 +1865,9 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>   	up_read(&ctx->map_changing_lock);
>   	uffd_move_unlock(dst_vma, src_vma);
>   out:
> -	VM_WARN_ON(moved < 0);
> -	VM_WARN_ON(err > 0);
> -	VM_WARN_ON(!moved && !err);
> +	VM_WARN_ON_ONCE(moved < 0);
> +	VM_WARN_ON_ONCE(err > 0);
> +	VM_WARN_ON_ONCE(!moved && !err);
>   	return moved ? moved : err;


Here you convert VM_WARN_ON to VM_WARN_ON_ONCE without stating it in the 
description (including the why).

> @@ -1956,9 +1954,9 @@ int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
>   	for_each_vma_range(vmi, vma, end) {
>   		cond_resched();
>   
> -		BUG_ON(!vma_can_userfault(vma, vm_flags, wp_async));
> -		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
> -		       vma->vm_userfaultfd_ctx.ctx != ctx);
> +		VM_WARN_ON_ONCE(!vma_can_userfault(vma, vm_flags, wp_async));
> +		VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx &&
> +				vma->vm_userfaultfd_ctx.ctx != ctx);
>   		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));

Which raises the question, why this here should still be a WARN

--
Cheers,

David / dhildenb