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

On Thu, Jan 11, 2024 at 09:55:36AM +0000, John Garry wrote:

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

quoted

On Wed, Jan 10, 2024 at 05:40:56PM -0800, Darrick J. Wong wrote:

quoted

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

I'm assuming O_CREAT also.

Yes.

quoted

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?

Sorry, this check should not be here at all, we should only check it
later.

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. 

True.  We might need to report the limits even without that, though.