Jeff Moyer <[email protected]> wrote:
>
> Currently, if you open a file O_DIRECT, truncate it to a size that is not a
> multiple of the disk block size, and then try to read the last block in the
> file, the read will return 0. The problem is in do_direct_IO, here:
>
> /* Handle holes */
> if (!buffer_mapped(map_bh)) {
> char *kaddr;
>
> ...
>
> if (dio->block_in_file >=
> i_size_read(dio->inode)>>blkbits) {
> /* We hit eof */
> page_cache_release(page);
> goto out;
> }
>
> We shift off any remaining bytes in the final block of the I/O, resulting
> in a 0-sized read. I've attached a patch that fixes this. I'm not happy
> about how ugly the math is getting, so suggestions are more than welcome.
>
> I've tested this with a simple program that performs the steps outlined for
> reproducing the problem above. Without the patch, we get a 0-sized result
> from read. With the patch, we get the correct return value from the short
> read.
OK. We do have some helper functions to make the math a little clearer.
How does this look?
--- devel/fs/direct-io.c~fix-o_direct-read-of-last-block-in-a-sparse-file 2006-01-23 15:42:31.000000000 -0800
+++ devel-akpm/fs/direct-io.c 2006-01-23 15:51:14.000000000 -0800
@@ -857,6 +857,7 @@ do_holes:
/* Handle holes */
if (!buffer_mapped(map_bh)) {
char *kaddr;
+ loff_t i_size_aligned;
/* AKPM: eargh, -ENOTBLK is a hack */
if (dio->rw == WRITE) {
@@ -864,8 +865,14 @@ do_holes:
return -ENOTBLK;
}
+ /*
+ * Be sure to account for a partial block as the
+ * last block in the file
+ */
+ i_size_aligned = ALIGN(i_size_read(dio->inode),
+ 1 << blkbits);
if (dio->block_in_file >=
- i_size_read(dio->inode)>>blkbits) {
+ i_size_aligned >> blkbits) {
/* We hit eof */
page_cache_release(page);
goto out;
_
-
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