[PATCH v2] io-mapping: Indicate mapping failure

[Ruhl, Michael J]

>-----Original Message-----
>From: Andrew Morton <akpm at linux-foundation.org>
>Sent: Tuesday, July 21, 2020 4:57 PM
>To: Ruhl, Michael J <michael.j.ruhl at intel.com>
>Cc: dri-devel at lists.freedesktop.org; Mike Rapoport <rppt at linux.ibm.com>;
>Andy Shevchenko <andriy.shevchenko at linux.intel.com>; Chris Wilson
><chris at chris-wilson.co.uk>; stable at vger.kernel.org
>Subject: Re: [PATCH v2] io-mapping: Indicate mapping failure
>
>On Tue, 21 Jul 2020 13:19:36 -0400 "Michael J. Ruhl"
><michael.j.ruhl at intel.com> wrote:
>
>> The !ATOMIC_IOMAP version of io_maping_init_wc will always return
>> success, even when the ioremap fails.
>>
>> Since the ATOMIC_IOMAP version returns NULL when the init fails, and
>> callers check for a NULL return on error this is unexpected.
>>
>> During a device probe, where the ioremap failed, a crash can look
>> like this:
>>
>> BUG: unable to handle page fault for address: 0000000000210000
>>  #PF: supervisor write access in kernel mode
>>  #PF: error_code(0x0002) - not-present page
>>  Oops: 0002 [#1] PREEMPT SMP
>>  CPU: 0 PID: 177 Comm:
>>  RIP: 0010:fill_page_dma [i915]
>>   gen8_ppgtt_create [i915]
>>   i915_ppgtt_create [i915]
>>   intel_gt_init [i915]
>>   i915_gem_init [i915]
>>   i915_driver_probe [i915]
>>   pci_device_probe
>>   really_probe
>>   driver_probe_device
>>
>> The remap failure occurred much earlier in the probe.  If it had
>> been propagated, the driver would have exited with an error.
>>
>> Return NULL on ioremap failure.
>>
>> ...
>>
>> --- a/include/linux/io-mapping.h
>> +++ b/include/linux/io-mapping.h
>> @@ -118,7 +118,7 @@ io_mapping_init_wc(struct io_mapping *iomap,
>>  	iomap->prot = pgprot_noncached(PAGE_KERNEL);
>>  #endif
>>
>> -	return iomap;
>> +	return iomap->iomem ? iomap : NULL;
>>  }
>>
>>  static inline void
>
>LGTM.  However I do think it would be stylistically better/typical to
>detect and handle the error early, rather than to blunder on,
>pointlessly initializing things?

Yeah, I pondered that, and then didn't do it...

>--- a/include/linux/io-mapping.h~io-mapping-indicate-mapping-failure-fix
>+++ a/include/linux/io-mapping.h
>@@ -107,9 +107,12 @@ io_mapping_init_wc(struct io_mapping *io
> 		   resource_size_t base,
> 		   unsigned long size)
> {
>+	iomap->iomem = ioremap_wc(base, size);
>+	if (!iomap->iomem)
>+		return NULL;
>+

This does make more sense.

I am confused by the two follow up emails I just got.

Shall I resubmit, or is this path (if !iomap->iomem) return NULL)
now in the tree. 😊

Thanks,

Mike

> 	iomap->base = base;
> 	iomap->size = size;
>-	iomap->iomem = ioremap_wc(base, size);
> #if defined(pgprot_noncached_wc) /* archs can't agree on a name ... */
> 	iomap->prot = pgprot_noncached_wc(PAGE_KERNEL);
> #elif defined(pgprot_writecombine)
>@@ -118,7 +121,7 @@ io_mapping_init_wc(struct io_mapping *io
> 	iomap->prot = pgprot_noncached(PAGE_KERNEL);
> #endif
>
>-	return iomap->iomem ? iomap : NULL;
>+	return iomap;
> }
>
> static inline void
>_