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

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Message-ID: <20240111050257.GA4457@lst.de>
Date: Thu, 11 Jan 2024 06:02:57 +0100
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Dave Chinner <david@...morbit.com>,
	John Garry <john.g.garry@...cle.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 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");