[PATCH] generic_file_buffered_write: handle partial DIO writes with multiple iovecs

This is a patch to generic_file_buffered_write() to correctly
handle partial O_DIRECT writes (because of unallocated blocks)
when there is more than 1 iovec.  Without this patch, the code is
writing the wrong iovec (it writes the first iovec a 2nd time).

Included is a test program dio_bug.c that shows the problem by:
	writing 4k to offset 4k
	writing 4k to offset 12k
	writing 8k to offset 4k
The result is that 8k write writes the 1st 4k of the buffer twice.

$ rm f; ./dio_bug f
wrong value offset 8k expected 0x33 got 0x11
wrong value offset 10k expected 0x44 got 0x22

with patch
$ rm f; ./dio_bug f

#define _GNU_SOURCE
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/uio.h>

main(int argc, char **argv)
{
	int fd;
	char *buf;
	int i;
	struct iovec v[2];

	fd = open(argv[1], O_DIRECT|O_RDWR|O_CREAT, 0666);

	if (fd < 0) {
		perror("open");
		exit(1);
	}

	buf = valloc(8192);

	lseek(fd, 0x1000, SEEK_SET);
	memset(buf, 0x11, 2048);
	memset(buf+2048, 0x22, 2048);
	i = write(fd, buf, 4096);	/* 4k write of 0x11 and 0x22 at 4k */

	lseek(fd, 0x3000, SEEK_SET);
	memset(buf, 0x55, 2048);
	memset(buf+2048, 0x66, 2048);
	i = write(fd, buf, 4096);	/* 4k write of 0x55 and 0x66 at 12k */

	lseek(fd, 0x1000, SEEK_SET);
	i = read(fd, buf, 4096);
	memset(buf+4096, 0x33 , 2048);
	memset(buf+4096+2048, 0x44 , 2048);

	v[0].iov_base = buf;
	v[0].iov_len = 4096;
	v[1].iov_base = buf + 4096;
	v[1].iov_len = 4096;
	lseek(fd, 0x1000, SEEK_SET);
	i = writev(fd, v, 2);	/* 8k write of 0x11, 0x22, 0x33, 0x44 at 4k */

	lseek(fd, 0x2000, SEEK_SET);
	i = read(fd, buf, 4096);
	if (buf[0] != 0x33)
		printf("wrong value offset 8k expected 0x33 got 0x%x\n",
			buf[0]);
	if (buf[2048] != 0x44)
		printf("wrong value offset 10k expected 0x44 got 0x%x\n",
			buf[2048]);

}
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -1933,7 +1933,16 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 
 	pagevec_init(&lru_pvec, 0);
 
-	buf = iov->iov_base + written;	/* handle partial DIO write */
+	/*
+	 * handle partial DIO write.  Adjust cur_iov if needed.
+	 */
+	if (likely(nr_segs == 1))
+		buf = iov->iov_base + written;
+	else {
+		filemap_set_next_iovec(&cur_iov, &iov_base, written);
+		buf = iov->iov_base + iov_base;
+	}
+
 	do {
 		unsigned long index;
 		unsigned long offset;