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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, May 20, 2025 at 02:42:09PM +0100, Usama Arif wrote:
> 
> 
> On 20/05/2025 14:34, Harry Yoo wrote:
> > 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@gmail.com>
> >> Reported-by: Vlad Poenaru <vlad.wing@gmail.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 ;)
> > 
> 
> Thanks for adding them to CC! I was just thinking of this as a memory
> allocation profiling issue and added the maintainers for it,
> but should have added slab maintainers as well.
> 
> 
> >>  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.
> > 
> 
> Isn't WARN_ONCE the same as pr_warn_once but with needing the condition
> of the first arg to be true? We only want to warn if alloc_slab_obj_exts
> returns non-zero. So WARN_ONCE should be ok?
> 

The difference is the impact on panic_on_warn users which are mostly
testing bots. This warning is not actionable, so I agree with Harry to
covert this to pr_warn_once().

> > 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
> >>
> >>
> > 
>