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Date: Thu, 11 Jan 2024 15:45:37 +0100
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, "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,
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	linux-nvme@lists.infradead.org, linux-xfs@vger.kernel.org,
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	linux-scsi@vger.kernel.org, ming.lei@redhat.com, bvanassche@acm.org,
	ojaswin@linux.ibm.com
Subject: Re: [PATCH v2 00/16] block atomic writes
Message-ID: <20240111144537.GA9295@lst.de>
References: <76c85021-dd9e-49e3-80e3-25a17c7ca455@oracle.com> <20231219151759.GA4468@lst.de> <fff50006-ccd2-4944-ba32-84cbb2dbd1f4@oracle.com> <20231221065031.GA25778@lst.de> <73d03703-6c57-424a-80ea-965e636c34d6@oracle.com> <ZZ3Q4GPrKYo91NQ0@dread.disaster.area> <20240110091929.GA31003@lst.de> <20240111014056.GL722975@frogsfrogsfrogs> <20240111050257.GA4457@lst.de> <d5db2291-36b4-4b22-89f2-1d9e7d30f0f1@oracle.com>
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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.