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Date: Thu, 11 Jan 2024 16:11:38 +0000
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Subject: Re: [PATCH v2 00/16] block atomic writes
To: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <djwong@kernel.org>, Dave Chinner
 <david@fromorbit.com>,
        axboe@kernel.dk, kbusch@kernel.org, sagi@grimberg.me,
        jejb@linux.ibm.com, martin.petersen@oracle.com,
        viro@zeniv.linux.org.uk, brauner@kernel.org, dchinner@redhat.com,
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From: John Garry <john.g.garry@oracle.com>
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> 
>>> 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