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

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Date:   Wed, 12 Apr 2017 09:27:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Pushkar Jambhlekar <pushkar.iit@...il.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] drivers/dax: Changing RC value

On Tue, Apr 11, 2017 at 1:58 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Tue, 11 Apr 2017 10:56:45 +0530 Pushkar Jambhlekar <pushkar.iit@...il.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@...il.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@...il.com>
    Signed-off-by: Dan Williams <dan.j.williams@...el.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);