[bug report] ext4 misses final i_size meta sync under O_DIRECT | O_SYNC semantics after iomap DIO conversion

[Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>]

Hi folks,

Our consumer reports a behavior change between pre-iomap and iomap
direct io conversion:

If the system crashes after an appending write to a file open with
O_DIRECT | O_SYNC flag set, file i_size won't be updated even if
O_SYNC was marked before.

It can be reproduced by a test program in the attachment with
gcc -o repro repro.c && ./repro testfile && echo c > /proc/sysrq-trigger

After some analysis, we found that before iomap direct I/O conversion,
the timing was roughly (taking Linux 3.10 codebase as an example):

	..
	- ext4_file_dio_write
	  - __generic_file_aio_write
	      ..
	    - ext4_direct_IO  # generic_file_direct_write
	      - ext4_ext_direct_IO
	        - ext4_ind_direct_IO  # final_size > inode->i_size
	          - ..
	          - ret = blockdev_direct_IO()
	          - i_size_write(inode, end) # orphan && ret > 0 &&
	                                   # end > inode->i_size
	          - ext4_mark_inode_dirty()
	          - ...
	  - generic_write_sync  # handling O_SYNC

So the dirty inode meta will be committed into journal immediately
if O_SYNC is set.  However, After commit 569342dc2485 ("ext4: move
inode extension/truncate code out from ->iomap_end() callback"),
the new behavior seems as below:

	..
	- ext4_dio_write_iter
	  - ext4_dio_write_checks  # extend = 1
	  - iomap_dio_rw
	      - __iomap_dio_rw
	      - iomap_dio_complete
	        - generic_write_sync
	  - ext4_handle_inode_extension  # extend = 1

So that i_size will be recorded only after generic_write_sync() is
called.  So O_SYNC won't flush the update i_size to the disk.

On the other side, after a quick look of XFS side, it will record
i_size changes in xfs_dio_write_end_io() so it seems that it doesn't
have this problem.

Thanks,
Gao Xiang

#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

#define PAGE_SIZE 4096

int test(char* file)
{
    char* buf = NULL;
    int ret = 0;
    int i = 0;
    posix_memalign((void**)(&buf), PAGE_SIZE, PAGE_SIZE);
    memset(buf, 0, PAGE_SIZE);
    for (i = 0 ; i < PAGE_SIZE ; ++i) buf[i] = i;
    int fd = open(file, O_WRONLY|O_CREAT|O_DIRECT|O_SYNC);
    if (fd == -1)
    {
        fprintf(stderr, "%s: %s\n", file, strerror(errno));
        exit(1);
    }
    struct stat st;
    ret = fstat(fd, &st);
    if (ret != 0)
    {
        fprintf(stderr, "%s: %s\n", file, strerror(errno));
        exit(1);
    }
    int offset = st.st_size;
    ret = pwrite(fd, buf, PAGE_SIZE, offset);
    if (ret != PAGE_SIZE)
    {
        fprintf(stderr, "write fail: ret %d %s\n", ret, strerror(errno));
    }
    close(fd);
    return 0;
}

int main(int argc, char ** argv)
{
    int ret = 0;
    char file[1024] = {};
    if (argc != 2)
    {
        fprintf(stderr, "usage: %s path-to-write\n", argv[0]);
        exit(2);
    }
    snprintf(file, sizeof(file), "%s_%d", argv[1], 0);
    test(file);
    return 0;
}