Re: [PATCH 4/6] s390/qeth: replace get_zeroed_page() with kzalloc()

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, May 28, 2026 at 10:09:52AM +0300, Mike Rapoport (Microsoft) wrote:
> qeth_get_trap_id() allocates a temporary buffer for STSI system
> information queries used to build trap identification strings.
> 
> This buffer can be allocated with kmalloc() as there's nothing special
> about it to go directly to the page allocator.
> 
> kmalloc() provides a better API that does not require ugly casts and
> kfree() does not need to know the size of the freed object.
> 
> Performance difference between kmalloc() and __get_free_pages() is not
> measurable as both allocators take an object/page from a per-CPU list for
> fast path allocations.
> 
> For the slow path the performance is anyway determined by the amount of
> reclaim involved rather than by what allocator is used.
> 
> Replace use of get_zeroed_page() with kzalloc() and free_page() with
> kfree().
> 
> Link: https://lore.kernel.org/all/635405e4-9423-4a25-a6e7-e03c8ea0bcbe@redhat.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  drivers/s390/net/qeth_core_main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/net/qeth_core_main.c b/drivers/s390/net/qeth_core_main.c
> index cf5f760d0e02..9274087557ec 100644
> --- a/drivers/s390/net/qeth_core_main.c
> +++ b/drivers/s390/net/qeth_core_main.c
> @@ -3362,7 +3362,7 @@ static int qeth_query_setdiagass(struct qeth_card *card)
>  
>  static void qeth_get_trap_id(struct qeth_card *card, struct qeth_trap_id *tid)
>  {
> -	unsigned long info = get_zeroed_page(GFP_KERNEL);
> +	unsigned long info = (unsigned long)kzalloc(PAGE_SIZE, GFP_KERNEL);
>  	struct sysinfo_2_2_2 *info222 = (struct sysinfo_2_2_2 *)info;
>  	struct sysinfo_3_2_2 *info322 = (struct sysinfo_3_2_2 *)info;
>  	struct ccw_dev_id ccwid;
> @@ -3381,7 +3381,7 @@ static void qeth_get_trap_id(struct qeth_card *card, struct qeth_trap_id *tid)
>  		EBCASC(info322->vm[0].name, sizeof(info322->vm[0].name));
>  		memcpy(tid->vmname, info322->vm[0].name, sizeof(tid->vmname));
>  	}
> -	free_page(info);
> +	kfree((void *)info);
>  }

Hm, another thing is that Sashiko claims that kmalloc allocations are not
necessarily correctly aligned when slab debugging or KASAN is enabled [1].

I believe that Sashiko is wrong, since Vlastimil addressed that with commit
59bb47985c1d ("mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two)")

Can you confirm that Sashiko is wrong?

[1] https://sashiko.dev/#/patchset/20260528-b4-s390-drivers-v1-0-b7108f54d722%40kernel.org?part=4