[PATCH RFC] xfs: set block device logical sector size on xfs_buftarg

[Eric Sandeen <sandeen@redhat.com>]

Pure RFC; this might be crazy.  Here's the problem I'm trying to solve:

Today, mkfs.xfs will select a 4k sector size for a 4k physical / 512 logical
drive.  (that change was done by me).  The thought was that it'd be an
efficiency gain to not make the drive do the (possible) RMW cycles on
512-byte log IO, primarily.

However, now this restricts all DIO to 4k alignment, not the otherwise-
possible 512.

This came up when qemu-kvm, in cache=none mode, tries to boot off an
image hosted on such a filesystem, and its bios wants to do a 512 byte
direct IO read off the disk - it fails.

But I'm wondering - the buftarg's bt_sshift and bt_smask are only used
in a few places.  

XFS_IOC_DIOINFO - to get the minimum io size
xfs_file_aio_read() and xfs_file_dio_aio_write() to check alignment
_xfs_buf_find() to be sure we aren't doing sub-sector IO.

So what I'm wondering is:  Can we somehow separate the "sector size"
that i.e. primarily the xfs log does its IO in, based on sb_sectsize,
from the actual, hard-minimum possible IO, in the buftarg bt_sshift &
bt_smask.


Something like this, though untested, and I'm probably missing something.

Our other option, I guess, is to just revert the mkfs change which
picks the physical rather than logical sector size, and go back to 
512 if it's available as a logical size.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---


diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 8968f50..58ce036 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -814,10 +814,12 @@ STATIC int
 xfs_setup_devices(
 	struct xfs_mount	*mp)
 {
+	xfs_buftarg_t		btp;
 	int			error;
 
+	btp = mp->m_ddev_targp;
 	error = xfs_setsize_buftarg(mp->m_ddev_targp, mp->m_sb.sb_blocksize,
-				    mp->m_sb.sb_sectsize);
+				    bdev_logical_block_size(btp));
 	if (error)
 		return error;
 
@@ -833,9 +835,10 @@ xfs_setup_devices(
 			return error;
 	}
 	if (mp->m_rtdev_targp) {
+		btp = mp->m_rtdev_targp;
 		error = xfs_setsize_buftarg(mp->m_rtdev_targp,
 					    mp->m_sb.sb_blocksize,
-					    mp->m_sb.sb_sectsize);
+					    bdev_logical_block_size(btp));
 		if (error)
 			return error;
 	}


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