linux-kernel - Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu affinity mask

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Message-ID: <87edhk2z03.fsf@cloudflare.com>
Date:   Tue, 24 Oct 2023 10:17:19 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com, Caleb Raitto <caraitto@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/2] virtio_pci: Don't make an extra copy of cpu
 affinity mask

On Tue, Oct 24, 2023 at 10:31 AM +08, Xuan Zhuo wrote:
> On Mon, 23 Oct 2023 18:52:45 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
>> On Thu, Oct 19, 2023 at 08:55 PM +08, Xuan Zhuo wrote:
>> > On Thu, 19 Oct 2023 12:16:24 +0200, Jakub Sitnicki <jakub@...udflare.com> wrote:
>> >> Since commit 19e226e8cc5d ("virtio: Make vp_set_vq_affinity() take a
>> >> mask.") it is actually not needed to have a local copy of the cpu mask.
>> >
>> >
>> > Could you give more info to prove this?
>
>
> Actually, my question is that can we pass a val on the stack(or temp value) to
> the irq_set_affinity_hint()?
>
> Such as the virtio-net uses zalloc_cpumask_var to alloc a cpu_mask, and
> that will be released.
>
>
>
> 	int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
> 				      bool setaffinity)
> 	{
> 		unsigned long flags;
> 		struct irq_desc *desc = irq_get_desc_lock(irq, &flags, IRQ_GET_DESC_CHECK_GLOBAL);
>
> 		if (!desc)
> 			return -EINVAL;
> ->		desc->affinity_hint = m;
> 		irq_put_desc_unlock(desc, flags);
> 		if (m && setaffinity)
> 			__irq_set_affinity(irq, m, false);
> 		return 0;
> 	}
> 	EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);
>
> The above code directly refers the mask pointer. If the mask is a temp value, I
> think that is a bug.

You are completely right. irq_set_affinity_hint stores the mask pointer.
irq_affinity_hint_proc_show later dereferences it when user reads out
/proc/irq/*/affinity_hint.

I have failed to notice that. That's why we need cpumask_copy to stay.

My patch is buggy. Please disregard.

I will send a v2 to only migrate from deprecated irq_set_affinity_hint.

> And I notice that many places directly pass the temp value to this API.
> And I am a little confused. ^_^ Or I missed something.

There seem two be two gropus of callers:

1. Those that use get_cpu_mask/cpumask_of/cpumask_of_node to produce a
   cpumask pointer which is a preallocated constant.

   $ weggli 'irq_set_affinity_hint(_, $func(_));' ~/src/linux

2. Those that pass a pointer to memory somewhere.

   $ weggli 'irq_set_affinity_hint(_, $mask);' ~/src/linux

(weggli tool can be found at https://github.com/weggli-rs/weggli)

I've looked over the callers from group #2 but I couldn't find any
passing a pointer memory on stack :-)

Thanks for pointing this out.

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