Re: [PATCH 2/2] drivers/dax: Changing RC value

Dan Williams
2017-04-12T16:27:18Z

On Tue, Apr 11, 2017 at 1:58 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Tue, 11 Apr 2017 10:56:45 +0530 Pushkar Jambhlekar <pushkar.iit@gmail.com> wrote:
>
>> Changing rc value from VM_FAULT_FALLBACK to VM_FAULT_SIGBUS for an unknown / unsupported fault size.
>>
>> ...
>>
>> --- a/drivers/dax/dax.c
>> +++ b/drivers/dax/dax.c
>> @@ -590,7 +590,7 @@ static int dax_dev_huge_fault(struct vm_fault *vmf,
>> rc = __dax_dev_pud_fault(dax_dev, vmf);
>> break;
>> default:
>> - rc = VM_FAULT_FALLBACK;
>> + rc = VM_FAULT_SIGBUS;
>> }
>> rcu_read_unlock();
>
> The change seems to make sense but more info would be helpful. What
> is wrong with the current code? ie, what goes wrong if we return
> VM_FAULT_FALLBACK here?

Hi Andrew, I took these two patches into my for-4.12/dax branch with
the following fixed up changelog:

commit 17756a4c681c320712798b783eb40006de9b4cc3
Author: Pushkar Jambhlekar <pushkar.iit@gmail.com>
Date: Tue Apr 11 09:12:25 2017 -0700

device-dax: fix dax_dev_huge_fault() unknown fault size handling

The default case for dax_dev_huge_fault() fault size handling mistakenly
returns when it should unlock. This is not a problem in practice since
the only three possible fault sizes are handled. Going forward, if the
core mm adds a new fault size beyond pte, pmd, or pud device-dax should
abort VM_FAULT_SIGBUS requests not VM_FAULT_FALLBACK since device-dax
guarantees a configured fault granularity for all faults.

Signed-off-by: Pushkar Jambhlekar <pushkar.iit@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --git a/drivers/dax/dax.c b/drivers/dax/dax.c
index 19795eb35579..94036d92ed16 100644
--- a/drivers/dax/dax.c
+++ b/drivers/dax/dax.c
@@ -591,7 +591,7 @@ static int dax_dev_huge_fault(struct vm_fault *vmf,
rc = __dax_dev_pud_fault(dax_dev, vmf);
break;
default:
- return VM_FAULT_FALLBACK;
+ rc = VM_FAULT_SIGBUS;
}
srcu_read_unlock(&dax_srcu, id);