xfs: fix locking for DAX writes (17879e8f) · Commits · 戴 / test

So far DAX writes inherited the locking from direct I/O writes, but the direct I/O model of using shared locks for writes is actually wrong for DAX. For direct I/O we're out of any standards and don't have to provide the Posix required exclusion between writers, but for DAX which gets transparently enable on applications without any knowledge of it we can't simply drop the requirement. Even worse this only happens for aligned writes and thus doesn't show up for many typical use cases. Signed-off-by:

Christoph Hellwig <hch@lst.de> Reviewed-by:

Dave Chinner <dchinner@redhat.com> Signed-off-by:

Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_file.c

+1 −19

Original line number	Diff line number	Diff line
		@@ -714,24 +714,11 @@ xfs_file_dax_write(
		struct address_space *mapping = iocb->ki_filp->f_mapping;
		struct inode *inode = mapping->host;
		struct xfs_inode *ip = XFS_I(inode);
		struct xfs_mount *mp = ip->i_mount;
		ssize_t ret = 0;
		int unaligned_io = 0;
		int iolock;
		int iolock = XFS_IOLOCK_EXCL;
		struct iov_iter data;

		/* "unaligned" here means not aligned to a filesystem block */
		if ((iocb->ki_pos & mp->m_blockmask) \|\|
		((iocb->ki_pos + iov_iter_count(from)) & mp->m_blockmask)) {
		unaligned_io = 1;
		iolock = XFS_IOLOCK_EXCL;
		} else if (mapping->nrpages) {
		iolock = XFS_IOLOCK_EXCL;
		} else {
		iolock = XFS_IOLOCK_SHARED;
		}
		xfs_rw_ilock(ip, iolock);

		ret = xfs_file_aio_write_checks(iocb, from, &iolock);
		if (ret)
		goto out;
		@@ -758,11 +745,6 @@ xfs_file_dax_write(
		WARN_ON_ONCE(ret);
		}

		if (iolock == XFS_IOLOCK_EXCL && !unaligned_io) {
		xfs_rw_ilock_demote(ip, XFS_IOLOCK_EXCL);
		iolock = XFS_IOLOCK_SHARED;
		}

		trace_xfs_file_dax_write(ip, iov_iter_count(from), iocb->ki_pos);

		data = *from;

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