On Mon, Feb 26, 2018 at 2:06 AM, Jan Kara <jack@xxxxxxx> wrote: > On Fri 23-02-18 16:43:27, Dan Williams wrote: >> Given that S_DAX is non-zero in the FS_DAX=n + DEV_DAX=y case, another >> mechanism besides the plain IS_DAX() check to compile out dead >> filesystem-dax code paths. Without IS_FSDAX() xfs will fail at link time >> with: >> >> ERROR: "dax_finish_sync_fault" [fs/xfs/xfs.ko] undefined! >> ERROR: "dax_iomap_fault" [fs/xfs/xfs.ko] undefined! >> ERROR: "dax_iomap_rw" [fs/xfs/xfs.ko] undefined! >> >> This compile failure was previously hidden by the fact that S_DAX was >> erroneously defined to '0' in the FS_DAX=n + DEV_DAX=y case. >> >> Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx> >> Cc: linux-xfs@xxxxxxxxxxxxxxx >> Cc: <stable@xxxxxxxxxxxxxxx> >> Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx> >> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> > > As much as I appreciate that relying on compiler to optimize out dead > branches results in nicer looking code this is an example where it > backfires. Also having IS_DAX() and IS_FSDAX() doing almost the same, just > not exactly the same, is IMHO a recipe for confusion (e.g. a casual reader > could think why does ext4 get away with using IS_DAX while XFS has to use > IS_FSDAX?). So I'd just prefer to handle this as is usual in other kernel > areas - define empty stubs for all exported functions when CONFIG_FS_DAX is > not enabled. That way code can stay without ugly ifdefs and we don't have > to bother with IS_FSDAX vs IS_DAX distinction in filesystem code. Thoughts? > I think my patch is incomplete either way, because the current IS_DAX() usages handle more than just compiling out calls to fs/dax.c symbols. I.e. even if there were stubs for all fs/dax.c call outs call there are still local usages of the helper. Lets kill IS_DAX() and only have IS_FSDAX() and IS_DEVDAX() with the S_ISCHR() check. Any issues with that?