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

[John Garry]

> > > 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.

Could we just error the SETXATTR ioctl when FS_XFLAG_FORCEALIGN is not 
set (and it is required)? The problem is that ioctl reports -EINVAL for 
any such errors, so hard for the user to know the issue...

Thanks,
John