linux-kernel - Re: [PATCH v3] kcov: don't generate a warning on vm_insert

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Date:   Mon, 4 Apr 2022 15:22:52 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aleksandr Nogikh <nogikh@...gle.com>
Cc:     kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, andreyknvl@...il.com, elver@...gle.com,
        glider@...gle.com, tarasmadan@...gle.com, bigeasy@...utronix.de
Subject: Re: [PATCH v3] kcov: don't generate a warning on vm_insert_page()'s
 failure

On Fri,  1 Apr 2022 18:25:12 +0000 Aleksandr Nogikh <nogikh@...gle.com> wrote:

> vm_insert_page()'s failure is not an unexpected condition, so don't do
> WARN_ONCE() in such a case.
> 
> Instead, print a kernel message and just return an error code.
> 
> ...
>
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -475,8 +475,11 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
>  	vma->vm_flags |= VM_DONTEXPAND;
>  	for (off = 0; off < size; off += PAGE_SIZE) {
>  		page = vmalloc_to_page(kcov->area + off);
> -		if (vm_insert_page(vma, vma->vm_start + off, page))
> -			WARN_ONCE(1, "vm_insert_page() failed");
> +		res = vm_insert_page(vma, vma->vm_start + off, page);
> +		if (res) {
> +			pr_warn_once("kcov: vm_insert_page() failed\n");
> +			return res;
> +		}
>  	}
>  	return 0;
>  exit:

Can you explain the rationale here?  If vm_insert_page() failure is an
expected condition, why warn at all?

I'm struggling to understand why a condition is worth a printk, but not
a WARN.

Some explanation of what leads to the vm_insert_page() failure would
have been helpful.