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

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Date: Thu, 11 Jan 2024 15:45:37 +0100
From: Christoph Hellwig <hch@....de>
To: John Garry <john.g.garry@...cle.com>
Cc: Christoph Hellwig <hch@....de>, "Darrick J. Wong" <djwong@...nel.org>,
	Dave Chinner <david@...morbit.com>, axboe@...nel.dk,
	kbusch@...nel.org, sagi@...mberg.me, jejb@...ux.ibm.com,
	martin.petersen@...cle.com, viro@...iv.linux.org.uk,
	brauner@...nel.org, dchinner@...hat.com, jack@...e.cz,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
	linux-scsi@...r.kernel.org, ming.lei@...hat.com, bvanassche@....org,
	ojaswin@...ux.ibm.com
Subject: 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:
>> 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.

Yes.

>> 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.