Re: [PATCH 1/2] block, dax: move "select DAX" from BLOCK to FS_DAX

[Dan Williams]

On Tue, May 9, 2017 at 12:57 AM, Geert Uytterhoeven
<geert@xxxxxxxxxxxxxx> wrote:
> Hi Dan,
>
> On Tue, May 9, 2017 at 12:36 AM, kbuild test robot <lkp@xxxxxxxxx> wrote:
>> [auto build test ERROR on linus/master]
>> [also build test ERROR on next-20170508]
>> [cannot apply to v4.11]
>> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>>
>> url:    https://github.com/0day-ci/linux/commits/Dan-Williams/block-dax-move-select-DAX-from-BLOCK-to-FS_DAX/20170509-051522
>> config: parisc-c3000_defconfig (attached as .config)
>> compiler: hppa-linux-gnu-gcc (Debian 6.1.1-9) 6.1.1 20160705
>> reproduce:
>>         wget https://raw.githubusercontent.com/01org/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>         chmod +x ~/bin/make.cross
>>         # save the attached .config to linux build tree
>>         make.cross ARCH=parisc
>>
>> All errors (new ones prefixed by >>):
>>
>>    fs/built-in.o: In function `bdev_dax_supported':
>>>> (.text.bdev_dax_supported+0x4c): undefined reference to `dax_get_by_host'
>>    fs/built-in.o: In function `bdev_dax_supported':
>>>> (.text.bdev_dax_supported+0x5c): undefined reference to `dax_read_lock'
>>    fs/built-in.o: In function `bdev_dax_supported':
>>>> (.text.bdev_dax_supported+0x7c): undefined reference to `dax_direct_access'
>>    fs/built-in.o: In function `bdev_dax_supported':
>>>> (.text.bdev_dax_supported+0x88): undefined reference to `dax_read_unlock'
>>    fs/built-in.o: In function `bdev_dax_supported':
>>>> (.text.bdev_dax_supported+0x90): undefined reference to `put_dax'
>
> I ran into the same issue if CONFIG_DAX=m (it's still selected by some other
> modular symbol). #if IS_ENABLED(CONFIG_DAX) is true in the modular case, so
> the dummies provided by include/linux/dax.h are not used.
>
> However, while changing it to #ifdef CONFIG_DAX allows to build vmlinux, it
> leads to other issues as DAX is compiled as a module:
>
>     drivers/dax/super.c:35: error: redefinition of âdax_read_lockâ
>     include/linux/dax.h:30: error: previous definition of
> âdax_read_lockâ was here
>
> Yes, calling into optional modular code from builtin code in fs/blockdev.c is
> tricky ;-(  Perhaps you can make bdev_dax_supported() a small wrapper
> that calls into the real code through a function pointer, when the DAX module
> is available?

In fact, that is close to what I did for v2, and it passed a full run
through the kbuild robot. I'll send it out shortly.