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Date: Thu, 11 Jan 2024 06:02:57 +0100
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Dave Chinner <david@fromorbit.com>,
	John Garry <john.g.garry@oracle.com>, axboe@kernel.dk,
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	martin.petersen@oracle.com, viro@zeniv.linux.org.uk,
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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: <20240111050257.GA4457@lst.de>
References: <20231219051456.GB3964019@frogsfrogsfrogs> <20231219052121.GA338@lst.de> <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>
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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);
> 
> ioctl(fd, FS_IOC_GETXATTR, &fsxattr);
> 
> fsxattr.fsx_xflags |= FS_XFLAG_FORCEALIGN | FS_XFLAG_WRITE_ATOMIC;
> fsxattr.fsx_extsize = 16384; /* only for hardware no-tears writes */
> 
> ioctl(fd, FS_IOC_SETXATTR, &fsxattr);
> 
> statx(fd, "", AT_EMPTY_PATH, STATX_ALL | STATX_WRITE_ATOMIC, &statx);
> 
> if (statx.stx_atomic_write_unit_max >= 16384) {
> 	pwrite(fd, &iov, 1, 0, RWF_SYNC | RWF_ATOMIC);
> 	printf("HAPPY DANCE\n");
> }

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();
}

fsxattr.fsx_xflags |= FS_XFLAG_WRITE_ATOMIC;
if (statx.stx_atomic_write_alignment) {
	fsxattr.fsx_xflags |= FS_XFLAG_FORCEALIGN;
	fsxattr.fsx_extsize = 16384; /* only for hardware no-tears writes */
}
if (ioctl(fd, FS_IOC_SETXATTR, &fsxattr) < 1) {
	bailout();
}

pwrite(fd, &iov, 1, 0, RWF_SYNC | RWF_ATOMIC);
printf("HAPPY DANCE\n");