linux-kernel - Re: [PATCH v2 13/22] iommufd: Add mmap interface

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Message-ID: <aBvHcyrLbQGerWFa@nvidia.com>
Date: Wed, 7 May 2025 13:49:55 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Baolu Lu <baolu.lu@...ux.intel.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 13/22] iommufd: Add mmap interface

On Wed, May 07, 2025 at 09:36:48AM -0300, Jason Gunthorpe wrote:
> On Tue, May 06, 2025 at 01:54:10PM -0700, Nicolin Chen wrote:
> 
> > Now I start to think about the FD situation: either a fault queue or
> > an eventq returns an FD and holds a refcount on the event object. So
> > the event object can't be destroyed unless the FD is released first.
> > Are we doing it incorrectly here?
> 
> Not necessarily, and maybe that is a good point that we are already
> doing these cross dependencies. But we could fix the FDs with some
> work too.

Noted it down. Similar to unmap_mapping_range() in the mmap case,
is close_fd() or file_close_fd() the one that we should use in the
event object destroy()? Either of which is holding a spinlock, so
likely it's safe from a close racing, v.s. the mmap situation?

> > I see! It just needs to call that function when we remove the mmap
> > for a vIOMMU destroy().
> 
> It is a little fussy to setup the inode as well, but yes.
> 
> The other small advantage is you don't need to setup a special VMA ops
> and do VMA tracking to hold the refcount on the vqueue object.
> 
> But there is also any annoying race with unmap in setting up the mmap
> PFNs which is why vfio is doing it from a fault handler..

I see. That's why VFIO is doing vmf_insert_pfn() in the fault op,
instead of calling remap_pfn_range() in mmap().

> So maybe refcount is the simplest option for now. We could fix it
> later and it won't prevent close() from working right.

Ack. I will keep the one that I posted in v3.

Thanks
Nicolin