[PATCH - 2.6.10] generic_file_buffered_write handle partial DIO writes with multiple iovecs

[Daniel McNeil]

Andrew,

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

Here's the patch:

--- linux-2.6.10.orig/mm/filemap.c      2005-01-18 15:32:52.531207134 -0800
+++ linux-2.6.10/mm/filemap.c   2005-01-18 15:32:09.252319333 -0800
@@ -1908,7 +1908,16 @@ generic_file_buffered_write(struct kiocb
 
        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;
 

Here is the test program:
#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]);
        
}


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