Re: [RFC PATCH v3 2/4] dax: Check for data cache aliasing at runtime

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-02-01 10:44, Mathieu Desnoyers wrote:
[...]
>> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
>> index 4e8fdcb3f1c8..b69c9e442cf4 100644
>> --- a/drivers/nvdimm/pmem.c
>> +++ b/drivers/nvdimm/pmem.c
>> @@ -560,17 +560,19 @@ static int pmem_attach_disk(struct device *dev,
>>       dax_dev = alloc_dax(pmem, &pmem_dax_ops);
>>       if (IS_ERR(dax_dev)) {
>>           rc = PTR_ERR(dax_dev);
>> -        goto out;
>> +        if (rc != -EOPNOTSUPP)
>> +            goto out;
> 
> If I compare the before / after this change, if previously
> pmem_attach_disk() was called in a configuration with FS_DAX=n, it would
> result in a NULL pointer dereference.

I was wrong. drivers/nvdimm/Kconfig has:

config BLK_DEV_PMEM
         select DAX

and

drivers/nvdimm/Makefile has:

obj-$(CONFIG_BLK_DEV_PMEM) += nd_pmem.o
nd_pmem-y := pmem.o

which means that anything in pmem.c can assume that alloc_dax() is
implemented.

[...]
>> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
>> index 4b7ecd4fd431..f911e58a24dd 100644
>> --- a/drivers/s390/block/dcssblk.c
>> +++ b/drivers/s390/block/dcssblk.c
>> @@ -681,12 +681,14 @@ dcssblk_add_store(struct device *dev, struct 
>> device_attribute *attr, const char
>>       if (IS_ERR(dev_info->dax_dev)) {
>>           rc = PTR_ERR(dev_info->dax_dev);
>>           dev_info->dax_dev = NULL;
>> -        goto put_dev;
>> +        if (rc != -EOPNOTSUPP)
>> +            goto put_dev;
> 
> config DCSSBLK selects FS_DAX_LIMITED and DAX.
> 
> I'm not sure what selecting DAX is trying to achieve here, because the
> Kconfig option is "FS_DAX".
> 
> So depending on the real motivation behind this select, we may want to
> consider failure rather than success in the -EOPNOTSUPP case.
> 

I missed that alloc_dax() is implemented as not supported based on
CONFIG_DAX (not CONFIG_FS_DAX).

Therefore DCSSBLK Kconfig does the right thing and always selects DAX,
and thus an implemented version of alloc_dax().

This takes care of two of my open questions at least. :)

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com