On Fri, Oct 17, 2014 at 03:49:39PM +0000, Mathieu Desnoyers wrote:
> > I kind of wonder if we shouldn't just declare the function. It's called
> > like this:
> >
> > if (IS_DAX(inode))
> > return dax_zero_page_range(inode, from, length,
> > ext4_get_block);
> > return __ext4_block_zero_page_range(handle, mapping, from, length);
> >
> > and if CONFIG_DAX is not set, IS_DAX evaluates to 0 at compile time, so
> > the compiler will optimise out the call to dax_zero_page_range() anyway.
>
> I strongly prefer to implement "unimplemented stub" as static inlines
> rather than defining to 0, because the compiler can check that the types
> passed to the function are valid, even in the #else configuration which
> uses the stubs.
I think my explanation was unclear. This is what I meant:
+++ b/include/linux/fs.h
@@ -2473,7 +2473,6 @@ extern loff_t fixed_size_llseek(struct file *file, loff_t
offset,
extern int generic_file_open(struct inode * inode, struct file * filp);
extern int nonseekable_open(struct inode * inode, struct file * filp);
-#ifdef CONFIG_FS_DAX
int dax_clear_blocks(struct inode *, sector_t block, long size);
int dax_zero_page_range(struct inode *, loff_t from, unsigned len, get_block_t)
;
int dax_truncate_page(struct inode *, loff_t from, get_block_t);
#define dax_mkwrite(vma, vmf, gb) dax_fault(vma, vmf, gb)
-#else
-static inline int dax_clear_blocks(struct inode *i, sector_t blk, long sz)
-{
- return 0;
-}
-
-static inline int dax_truncate_page(struct inode *i, loff_t frm, get_block_t gb)
-{
- return 0;
-}
-
-static inline int dax_zero_page_range(struct inode *i, loff_t frm,
- unsigned len, get_block_t gb)
-{
- return 0;
-}
-
-static inline ssize_t dax_do_io(int rw, struct kiocb *iocb,
- struct inode *inode, struct iov_iter *iter, loff_t pos,
- get_block_t get_block, dio_iodone_t end_io, int flags)
-{
- return -ENOTTY;
-}
-#endif
#ifdef CONFIG_BLOCK
typedef void (dio_submit_t)(int rw, struct bio *bio, struct inode *inode,
So after the preprocessor has run, the compiler will see:
if (0)
return dax_zero_page_range(inode, from, length, ext4_get_block);
and it will still do type checking on the call, even though it will eliminate
the call.
I think what you're really complaining about is that the argument to
IS_DAX() is not checked for being an inode.
We could solve that this way:
#ifdef CONFIG_FS_DAX
#define S_DAX 8192
#else
#define S_DAX 0
#endif
...
#define IS_DAX(inode) ((inode)->i_flags & S_DAX)
After preprocessing, the compiler than sees:
if (((inode)->i_flags & 0))
return dax_zero_page_range(inode, from, length, ext4_get_block);
and successfully deduces that the condition evaluates to 0, and still
elide the reference to dax_zero_page_range (checked with 'nm').
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