linux-kernel - Re: [RFT][PATCH v2 1/9] vfio: Make vfio_unpin

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Date:   Thu, 7 Jul 2022 12:38:46 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Tian, Kevin" <kevin.tian@...el.com>,
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Subject: Re: [RFT][PATCH v2 1/9] vfio: Make vfio_unpin_pages() return void

On Thu, Jul 07, 2022 at 04:22:10PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 07, 2022 at 10:12:41AM -0700, Nicolin Chen wrote:
> > On Thu, Jul 07, 2022 at 08:42:28AM +0000, Tian, Kevin wrote:
> > > External email: Use caution opening links or attachments
> > > 
> > > 
> > > > From: Nicolin Chen <nicolinc@...dia.com>
> > > > Sent: Wednesday, July 6, 2022 2:28 PM
> > > >
> > > > There's only one caller that checks its return value with a WARN_ON_ONCE,
> > > > while all other callers do not check return value at all. So simplify the
> > > > API to return void by embedding similar WARN_ON_ONCEs.
> > > 
> > > While this change keeps the similar effect as before it leads to different
> > > policy for same type of errors between pin and unpin paths:
> > 
> > I think it's because of the policy that an undo function should not
> > fail. Meanwhile, indulging faulty inputs isn't good either.
> > 
> > > e.g.
> > > 
> > > vfio_unpin_pages():
> > >         if (WARN_ON_ONCE(!user_pfn || !npage || !vfio_assert_device_open(device)))
> > >                 return;
> > > 
> > > vfio_pin_pages():
> > >         if (!user_pfn || !phys_pfn || !npage ||
> > >             !vfio_assert_device_open(device))
> > >                 return -EINVAL;
> > > 
> > > It sounds a bit weird when reading related code...
> > 
> > Any better way to handle this?
> 
> They should all be WARN_ON's, that is the standard pattern to assert
> that function arguments must be correctly formed.

OK. I can change that. I assume that, not confined to arguments,
we might want to have a WARN_ON for the return value check also.

> I would also drop the tests that obviously will oops on their on
> anyone, like NULL pointer checks. This is a semi-performance path.

OK. I will simply remove those NULL pointer checks. Actually,
that !user_pfn check is gone anyway in the following patch, as
user_pfn is replaced with iova.