Re: [PATCH] dax: fix for do not print error message for non-persistent memory block device

[Coly Li <colyli@suse.de>]

On 2020/9/4 00:06, Ira Weiny wrote:
> On Thu, Sep 03, 2020 at 07:55:49PM +0800, Coly Li wrote:
>> When calling __generic_fsdax_supported(), a dax-unsupported device may
>> not have dax_dev as NULL, e.g. the dax related code block is not enabled
>> by Kconfig.
>>
>> Therefore in __generic_fsdax_supported(), to check whether a device
>> supports DAX or not, the following order should be performed,
>> - If dax_dev pointer is NULL, it means the device driver explicitly
>>   announce it doesn't support DAX. Then it is OK to directly return
>>   false from __generic_fsdax_supported().
>> - If dax_dev pointer is NOT NULL, it might be because the driver doesn't
>>   support DAX and not explicitly initialize related data structure. Then
>>   bdev_dax_supported() should be called for further check.
>>
>> IMHO if device driver desn't explicitly set its dax_dev pointer to NULL,
>> this is not a bug. Calling bdev_dax_supported() makes sure they can be
>> recognized as dax-unsupported eventually.
>>
>> This patch does the following change for the above purpose,
>>     -       if (!dax_dev && !bdev_dax_supported(bdev, blocksize)) {
>>     +       if (!dax_dev || !bdev_dax_supported(bdev, blocksize)) {
>>
>>
>> Fixes: c2affe920b0e ("dax: do not print error message for non-persistent memory block device")
>> Signed-off-by: Coly Li <colyli@suse.de>
> 
> I hate to do this because I realize this is a bug which people really need
> fixed.
> 
> However, shouldn't we also check (!dax_dev || !bdev_dax_supported()) as the
> _first_ check in __generic_fsdax_supported()?
> 
> It seems like the other pr_info's could also be called when DAX is not
> supported and we probably don't want them to be?
> 
> Perhaps that should be a follow on patch though.  So...

I am not author of c2affe920b0e, but I guess it was because
bdev_dax_supported() needed blocksize, so blocksize should pass previous
checks firstly to make sure bdev_dax_supported() has a correct blocksize
to check.

> 
> As a direct fix to c2affe920b0e
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>

Thanks.

Coly Li


> 
>> Cc: Adrian Huang <ahuang12@lenovo.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: Jan Kara <jack@suse.com>
>> Cc: Mike Snitzer <snitzer@redhat.com>
>> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
>> Cc: Vishal Verma <vishal.l.verma@intel.com>
>> ---
>>  drivers/dax/super.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
>> index 32642634c1bb..e5767c83ea23 100644
>> --- a/drivers/dax/super.c
>> +++ b/drivers/dax/super.c
>> @@ -100,7 +100,7 @@ bool __generic_fsdax_supported(struct dax_device *dax_dev,
>>  		return false;
>>  	}
>>  
>> -	if (!dax_dev && !bdev_dax_supported(bdev, blocksize)) {
>> +	if (!dax_dev || !bdev_dax_supported(bdev, blocksize)) {
>>  		pr_debug("%s: error: dax unsupported by block device\n",
>>  				bdevname(bdev, buf));
>>  		return false;
>> -- 
>> 2.26.2
>>
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