Re: [PATCH 2/6] s390/dasd: replace get_zeroed_page() with kzalloc()

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

On Thu, May 28, 2026 at 10:09:50AM +0300, Mike Rapoport (Microsoft) wrote:
> DASD driver uses get_zeroed_page() to allocate pages for the Extended Error
> Reporting software ring buffer and for a scratch buffer for formatting
> sense dump diagnostic text.
> 
> These buffers 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/block/dasd_eckd.c | 12 ++++++------
>  drivers/s390/block/dasd_eer.c  |  4 ++--
>  2 files changed, 8 insertions(+), 8 deletions(-)

...

>  static void dasd_eckd_dump_sense(struct dasd_device *device,
> @@ -6958,7 +6958,7 @@ dasd_eckd_init(void)
>  		kfree(pe_handler_worker);
>  		kfree(dasd_reserve_req);
>  		kfree(dasd_vol_info_req);
> -		free_page((unsigned long)rawpadpage);
> +		kfree(rawpadpage);
>  	}
>  	return ret;
>  }
> @@ -6969,7 +6969,7 @@ dasd_eckd_cleanup(void)
>  	ccw_driver_unregister(&dasd_eckd_driver);
>  	kfree(pe_handler_worker);
>  	kfree(dasd_reserve_req);
> -	free_page((unsigned long)rawpadpage);
> +	kfree(rawpadpage);
>  }

This is not correct. The allocation is still done with __get_free_page().

I'm not sure about the whole approach / effort, since this allows for subtle
bugs where pages are allocated and freed at non-obvious locations. All of that
works now, but these conversions may lead to subtle bugs (this particular one
is not subtle and easy to spot).

Is the net gain of this conversion really worth it?