Re: [linux-next:master 5601/6082] drivers/md/dm.c:2131:undefined reference to `set_dax_nocache'

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

On 2024-02-13 00:12, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> head:   ae00c445390b349e070a64dc62f08aa878db7248
> commit: 9af4599a77df42dba7f32c76a97a19f5703f44b7 [5601/6082] dm: cleanup alloc_dax() error handling
> config: openrisc-randconfig-r011-20221209 (https://download.01.org/0day-ci/archive/20240213/202402131351.a0FZOgEG-lkp@intel.com/config)
> compiler: or1k-linux-gcc (GCC) 13.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240213/202402131351.a0FZOgEG-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202402131351.a0FZOgEG-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>     or1k-linux-ld: drivers/md/dm.o: in function `alloc_dev':
>>> drivers/md/dm.c:2131:(.text+0x9f34): undefined reference to `set_dax_nocache'
>>> drivers/md/dm.c:2131:(.text+0x9f34): relocation truncated to fit: R_OR1K_INSN_REL_26 against undefined symbol `set_dax_nocache'
>>> or1k-linux-ld: drivers/md/dm.c:2132:(.text+0x9f3c): undefined reference to `set_dax_nomc'
>>> drivers/md/dm.c:2132:(.text+0x9f3c): relocation truncated to fit: R_OR1K_INSN_REL_26 against undefined symbol `set_dax_nomc'
>     pahole: .tmp_vmlinux.btf: Invalid argument
>     .btf.vmlinux.bin.o: file not recognized: file format not recognized

It comes as a surprise that a call to set_dax_nocache() from within a
branch provably never taken ends up keeping a reference to the missing
symbol. I would have expected compiler optimizations to remove the call
to the missing symbol altogether, but maybe I'm expecting too much.

Basically here we have:

CONFIG_DAX=n

static inline struct dax_device *alloc_dax(void *private,
                 const struct dax_operations *ops)
{
         return ERR_PTR(-EOPNOTSUPP);
}

static struct mapped_device *alloc_dev(int minor)
{
[...]
         dax_dev = alloc_dax(md, &dm_dax_ops);
         if (IS_ERR(dax_dev)) {
                 if (PTR_ERR(dax_dev) != -EOPNOTSUPP)
                         goto bad;
         } else {
                 set_dax_nocache(dax_dev);
                 set_dax_nomc(dax_dev);
                 md->dax_dev = dax_dev;
                 if (dax_add_host(dax_dev, md->disk))
                         goto bad;
         }

The simple fix would be something like this, but I would like to
understand why we observe this specifically on openrisc and loongarch
(https://lore.kernel.org/oe-kbuild-all/202402140037.wGfA1kqX-lkp@intel.com/).

diff --git a/include/linux/dax.h b/include/linux/dax.h
index df2d52b8a245..9d3e3327af4c 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -63,6 +63,8 @@ void kill_dax(struct dax_device *dax_dev);
  void dax_write_cache(struct dax_device *dax_dev, bool wc);
  bool dax_write_cache_enabled(struct dax_device *dax_dev);
  bool dax_synchronous(struct dax_device *dax_dev);
+void set_dax_nocache(struct dax_device *dax_dev);
+void set_dax_nomc(struct dax_device *dax_dev);
  void set_dax_synchronous(struct dax_device *dax_dev);
  size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
  		void *addr, size_t bytes, struct iov_iter *i);
@@ -105,6 +107,12 @@ static inline bool dax_synchronous(struct dax_device *dax_dev)
  {
  	return true;
  }
+static inline void set_dax_nocache(struct dax_device *dax_dev)
+{
+}
+static inline void set_dax_nomc(struct dax_device *dax_dev)
+{
+}
  static inline void set_dax_synchronous(struct dax_device *dax_dev)
  {
  }
@@ -120,9 +128,6 @@ static inline size_t dax_recovery_write(struct dax_device *dax_dev,
  }
  #endif
  
-void set_dax_nocache(struct dax_device *dax_dev);
-void set_dax_nomc(struct dax_device *dax_dev);
-
  struct writeback_control;
  #if defined(CONFIG_BLOCK) && defined(CONFIG_FS_DAX)
  int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk);

Thanks,

Mathieu


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