linux-kernel - Re: [PATCH v2][next] x86/uv/time: Replace one-element array and save heap space

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Date:   Thu, 01 Oct 2020 08:39:39 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Steve Wahl <steve.wahl@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, platform-driver-x86@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] x86/uv/time: Replace one-element array and
 save heap space

On Thu, 2020-10-01 at 09:56 -0500, Gustavo A. R. Silva wrote:
> There is a regular need in the kernel to provide a way to declare having
> a dynamically sized set of trailing elements in a structure. Kernel code
> should always use “flexible array members”[1] for these cases. The older
> style of one-element or zero-length arrays should no longer be used[2].
> 
> struct uv_rtc_timer_head contains a one-element array cpu[1].
> 
> Switch it to a flexible array and use the struct_size() helper to
> calculate the allocation size. Also, save some heap space in the
> process[3].

trivia:

> diff --git a/arch/x86/platform/uv/uv_time.c b/arch/x86/platform/uv/uv_time.c
[]
> @@ -148,9 +148,8 @@ static __init int uv_rtc_allocate_timers(void)
>  		struct uv_rtc_timer_head *head = blade_info[bid];
>  
>  		if (!head) {
> -			head = kmalloc_node(sizeof(struct uv_rtc_timer_head) +
> -				(uv_blade_nr_possible_cpus(bid) *
> -					2 * sizeof(u64)),
> +			head = kmalloc_node(struct_size(head, cpu,
> +				uv_blade_nr_possible_cpus(bid)),
>  				GFP_KERNEL, nid);
>  			if (!head) {
>  				uv_rtc_deallocate_timers();

Maybe save the value of uv_blade_nr_possible_cpus(bid)
to reduce duplication and make the sizeof_struct more
readable?

		if (!head) {
			int ncpus = uv_blade_nr_possible_cpus(bid);

			head = kmalloc_node(struct_size(head, cpu, ncpus),
					    GFP_KERNEL, nid);
			if (!head) {
				uv_rtc_deallocate_timers();
				return -ENOMEM;
			}
			spin_lock_init(&head->lock);
			head->ncpus = ncpus;
			head->next_cpu = -1;
			blade_info[bid] = head;
		}