linux-kernel - Re: [PATCH v2 04/10] iommu/arm-smmu-v3: Allocate IOTLB cache tag if no id to reuse

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Message-ID: <aXfpaN4UCYcY9W8T@Asurada-Nvidia>
Date: Mon, 26 Jan 2026 14:23:36 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<jpb@...nel.org>, <praan@...gle.com>, <miko.lenczewski@....com>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v2 04/10] iommu/arm-smmu-v3: Allocate IOTLB cache tag if
 no id to reuse

On Mon, Jan 26, 2026 at 05:06:40PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 21, 2026 at 05:24:22PM -0800, Nicolin Chen wrote:
> > @@ -3220,6 +3241,9 @@ arm_smmu_master_build_inv(struct arm_smmu_master *master,
> >  		break;
> >  	}
> >  
> > +	/* Set a default users counter */
> > +	refcount_set(&cur->users, 1);
> 
> I think abusing users here is a little too hard to read..
> 
> Can we just keep track in the state somehow with a flag?
>
> Or maybe union in a "bool needs_free" that is for the on-stack version
> of this structure?

This doesn't only apply to the case when a tag is newly allocated
during the attach, but it can be used when an old tag has users=0
during a detach, right?

Hmm, perhaps I should have just followed your suggestion, letting
arm_smmu_invs_unref() set the users at all:
https://lore.kernel.org/all/20251219170551.GF254720@nvidia.com/

FWIW, I am hoping to squash this into the base series (v10), so we
wouldn't add a free_fn function pointer in the base series and then
remove it in this followup series.

Nicolin