Hi,
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.
Thanks,
Jeff
Signed-off-by: Jeff Moyer <[email protected]>
--- linux-2.6.16-rc1-mm1/fs/direct-io.c.orig 2006-01-19 15:45:50.000000000 -0500
+++ linux-2.6.16-rc1-mm1/fs/direct-io.c 2006-01-19 16:09:33.000000000 -0500
@@ -857,6 +857,7 @@ do_holes:
/* Handle holes */
if (!buffer_mapped(map_bh)) {
char *kaddr;
+ loff_t i_size = i_size_read(dio->inode);
/* AKPM: eargh, -ENOTBLK is a hack */
if (dio->rw == WRITE) {
@@ -864,8 +865,10 @@ do_holes:
return -ENOTBLK;
}
- if (dio->block_in_file >=
- i_size_read(dio->inode)>>blkbits) {
+ /* Be sure to account for a partial block as
+ * the last block in the file */
+ if (dio->block_in_file >= i_size >> blkbits &&
+ !(i_size & ~((1<<blkbits)-1))) {
/* We hit eof */
page_cache_release(page);
goto out;
-
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