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

John Garry <john.g.garry@xxxxxxxxxx> · Thu, 11 Jan 2024 09:55:36 +0000

On 11/01/2024 05:02, Christoph Hellwig wrote:
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);

I'm assuming O_CREAT also.

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();
}

How could this value be >= 16384 initially? Would it be from 
pre-configured FS alignment, like XFS RT extsize? Or is this from some 
special CoW-based atomic write support? Or FS block size of 16384?

Incidentally, for consistency only setting FS_XFLAG_WRITE_ATOMIC will 
lead to FMODE_CAN_ATOMIC_WRITE being set. So until FS_XFLAG_WRITE_ATOMIC 
is set would it make sense to have statx return 0 for 
STATX_WRITE_ATOMIC. Otherwise the user may be misled to think that it is 
ok to issue an atomic write (when it isn’t).

Thanks,
John

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");