Re: [PATCH v2 00/16] block atomic writes

[Christoph Hellwig <hch@lst.de>]

On Wed, Jan 10, 2024 at 05:40:56PM -0800, Darrick J. Wong wrote:
> struct statx statx;
> struct fsxattr fsxattr;
> int fd = open('/foofile', O_RDWR | O_DIRECT);
> 
> ioctl(fd, FS_IOC_GETXATTR, &fsxattr);
> 
> fsxattr.fsx_xflags |= FS_XFLAG_FORCEALIGN | FS_XFLAG_WRITE_ATOMIC;
> fsxattr.fsx_extsize = 16384; /* only for hardware no-tears writes */
> 
> ioctl(fd, FS_IOC_SETXATTR, &fsxattr);
> 
> statx(fd, "", AT_EMPTY_PATH, STATX_ALL | STATX_WRITE_ATOMIC, &statx);
> 
> if (statx.stx_atomic_write_unit_max >= 16384) {
> 	pwrite(fd, &iov, 1, 0, RWF_SYNC | RWF_ATOMIC);
> 	printf("HAPPY DANCE\n");
> }

I think this still needs a check if the fs needs alignment for
atomic writes at all. i.e.

struct statx statx;
struct fsxattr fsxattr;
int fd = open('/foofile', O_RDWR | O_DIRECT);

ioctl(fd, FS_IOC_GETXATTR, &fsxattr);
statx(fd, "", AT_EMPTY_PATH, STATX_ALL | STATX_WRITE_ATOMIC, &statx);
if (statx.stx_atomic_write_unit_max < 16384) {
	bailout();
}

fsxattr.fsx_xflags |= FS_XFLAG_WRITE_ATOMIC;
if (statx.stx_atomic_write_alignment) {
	fsxattr.fsx_xflags |= FS_XFLAG_FORCEALIGN;
	fsxattr.fsx_extsize = 16384; /* only for hardware no-tears writes */
}
if (ioctl(fd, FS_IOC_SETXATTR, &fsxattr) < 1) {
	bailout();
}

pwrite(fd, &iov, 1, 0, RWF_SYNC | RWF_ATOMIC);
printf("HAPPY DANCE\n");