linux-kernel - Re: [PATCH v2 08/22] iommufd: Abstract iopt_pin_pages and iopt_unpin

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Message-ID: <aA+92fNNbDI3Qowk@Asurada-Nvidia>
Date: Mon, 28 Apr 2025 10:41:45 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <corbet@....net>,
	<will@...nel.org>, <bagasdotme@...il.com>, <robin.murphy@....com>,
	<joro@...tes.org>, <thierry.reding@...il.com>, <vdumpa@...dia.com>,
	<jonathanh@...dia.com>, <shuah@...nel.org>, <jsnitsel@...hat.com>,
	<nathan@...nel.org>, <peterz@...radead.org>, <yi.l.liu@...el.com>,
	<mshavit@...gle.com>, <praan@...gle.com>, <zhangzekun11@...wei.com>,
	<iommu@...ts.linux.dev>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-tegra@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<patches@...ts.linux.dev>, <mochs@...dia.com>, <alok.a.tiwari@...cle.com>,
	<vasant.hegde@....com>
Subject: Re: [PATCH v2 08/22] iommufd: Abstract iopt_pin_pages and
 iopt_unpin_pages helpers

On Sun, Apr 27, 2025 at 03:22:13PM +0800, Baolu Lu wrote:
> On 4/26/25 13:58, Nicolin Chen wrote:
> > The new vCMDQ object will be added for HW to access the guest memory for a
> > HW-accelerated virtualization feature. It needs to ensure the guest memory
> > pages are pinned when HW accesses them and they are contiguous in physical
> > address space.
> > 
> > This is very like the existing iommufd_access_pin_pages() that outputs the
> > pinned page list for the caller to test its contiguity.
> > 
> > Move those code from iommufd_access_pin/unpin_pages() and related function
> > for a pair of iopt helpers that can be shared with the vCMDQ allocator. As
> > the vCMDQ allocator will be a user-space triggered ioctl function, WARN_ON
> > would not be a good fit in the new iopt_unpin_pages(), thus change them to
> > use WARN_ON_ONCE instead.
> 
> I'm uncertain, but perhaps pr_warn_ratelimited() would be a better
> alternative to WARN_ON() here? WARN_ON_ONCE() generates warning messages
> with kernel call traces in the kernel messages, which might lead users
> to believe that something serious has happened in the kernel.

We already have similar practice, e.g. iommufd_hwpt_nested_alloc.

In my review, a WARN_ON/WARN_ON_ONCE means there is a kernel bug,
which shouldn't occur in the first place and isn't something that
user space should concern. In case that it is hit, a WARN_ON_ONCE
only spits one piece of traces that is enough for kernel folks to
identify what's wrong, while pr_warn_ratelimited would likely end
up with periodical warnings (more lines) that are neither related
to user space nor useful for kernel.

Thanks
Nicolin