linux-kernel - Re: [PATCH 2/2] mm: slub: only warn once when allocating slab obj extensions fails

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Message-ID: <aCyEyxHEXQ7DU9I1@harry>
Date: Tue, 20 May 2025 22:34:03 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, surenb@...gle.com,
        hannes@...xchg.org, shakeel.butt@...ux.dev, vlad.wing@...il.com,
        linux-mm@...ck.org, kent.overstreet@...ux.dev,
        linux-kernel@...r.kernel.org, kernel-team@...a.com, vbabka@...e.cz,
        cl@...two.org, rientjes@...gle.com, roman.gushchin@...ux.dev
Subject: Re: [PATCH 2/2] mm: slub: only warn once when allocating slab obj
 extensions fails

On Tue, May 20, 2025 at 01:25:47PM +0100, Usama Arif wrote:
> In memory bound systems, a large number of warnings for failing this
> allocation repeatedly may mask any real issues in the system
> during memory pressure being reported in dmesg. Change this to
> WARN_ONCE.
> 
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> Reported-by: Vlad Poenaru <vlad.wing@...il.com>
> Closes: https://lore.kernel.org/all/17fab2d6-5a74-4573-bcc3-b75951508f0a@gmail.com/
> ---

Hi,

Please Cc SLAB ALLOCATOR folks in MAINTAINERS on patches that touch
slab code ;)

>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index bf43c403ead2..97cb3d9e8d00 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2102,7 +2102,7 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
>  
>  	slab = virt_to_slab(p);
>  	if (!slab_obj_exts(slab) &&
> -	    WARN(alloc_slab_obj_exts(slab, s, flags, false),
> +	    WARN_ONCE(alloc_slab_obj_exts(slab, s, flags, false),
>  		 "%s, %s: Failed to create slab extension vector!\n",
>  		 __func__, s->name))

I think this should be pr_warn_once()?
I'm not sure why this was WARN() in the first place.

The coding style guide explicitly states that:
> Do not WARN lightly
> ===================
>
> WARN*() is intended for unexpected, this-should-never-happen situations.
> WARN*() macros are not to be used for anything that is expected to happen
> during normal operation. These are not pre- or post-condition asserts,
> for example. Again: WARN*() must not be used for a condition that is
> expected to trigger easily, for example, by user space actions.
> pr_warn_once() is a possible alternative, if you need to notify the user
> of a problem.

And failing to allocate the extension vector can happen during normal
operations.

panic_on_warn users will be unhappy if they notice their kernel panicked
just because their kernel failed to allocate slab extension vectors, which is
a totally normal situtation.

>  		return NULL;
> -- 
> 2.47.1
> 
> 

-- 
Cheers,
Harry / Hyeonggon