From: Jeff Moyer 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. Signed-off-by: Jeff Moyer Signed-off-by: Andrew Morton --- fs/direct-io.c | 9 ++++++++- 1 files changed, 8 insertions(+), 1 deletion(-) diff -puN fs/direct-io.c~fix-o_direct-read-of-last-block-in-a-sparse-file fs/direct-io.c --- 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; _