Re: [PATCH 05/13] xfs: add a _require_xfs_data_csum helper

[Christoph Hellwig <hch@lst.de>]

On Mon, Oct 05, 2026 at 07:48:56AM -0700, Darrick J. Wong wrote:
> Oh wow, a new ioctl.  Having not tried to do anything with it, it looks
> promising.  Would it be useful for a filesystem with software checksums
> to advertise something like this:
> 
> struct logical_block_metadata_cap foo = {
> 	.lbmd_flags = LBMD_FS_CSUM_CRC32C, /* doesn't yet exist */
> 	.lbmd_interval = i_blocksize(...),
> 	.lbmd_size = 4,
> };

Yes, see the patch below for that below, It might or might not apply to
the current series, but it should give you the idea.

> Even though userspace cannot (yet) access the per-fsblock crc32c data?

Well, I want that to be possible eventually.  I even have working code,
but it has too many layering violations to publish it at the moment.

> Also, if the fs supports per-fsblock checksums and the storage supports
> per-LBA PI, which gets advertised?

Wherever the file systems wants to store it because it intercepts both
the ioctl and the io_uring with metadata ops. So probably is it's own
location.  I should probably add a test for xfs crcs on top of PI to
make sure nothing breaks this..

---
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 96ca3e480cb9..6aa3e7f78eb5 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -46,6 +46,7 @@
 #include "xfs_verify_media.h"
 #include "xfs_zone_priv.h"
 #include "xfs_zone_alloc.h"
+#include "xfs_rtcsum.h"
 
 #include <linux/mount.h>
 #include <linux/fileattr.h>
@@ -1200,6 +1201,36 @@ xfs_ioctl_fs_counts(
 	return 0;
 }
 
+static int
+xfs_ioc_getlbmd_cap(
+	struct xfs_inode		*ip,
+	unsigned int			cmd,
+	struct logical_block_metadata_cap __user *argp)
+{
+	struct xfs_mount		*mp = ip->i_mount;
+	size_t				usize = _IOC_SIZE(cmd);
+	struct logical_block_metadata_cap lbm = {};
+
+	if (xfs_is_rtcsum_inode(ip)) {
+		lbm.lbmd_flags |= LBMD_PI_CAP_INTEGRITY;
+		lbm.lbmd_interval = mp->m_sb.sb_blocksize;
+		lbm.lbmd_pi_size = 1U << mp->m_rtcsum_shift;
+		lbm.lbmd_size = lbm.lbmd_pi_size;
+		switch (mp->m_sb.sb_rtcsum) {
+		case XFS_CSUM_TYPE_CRC32C:
+			lbm.lbmd_guard_tag_type = LBMD_PI_CSUM_CRC32C;
+			break;
+		case XFS_CSUM_TYPE_CRC64:
+			lbm.lbmd_guard_tag_type = LBMD_PI_CSUM_CRC64_NVME;
+			break;
+		default:
+			WARN_ON_ONCE(1);
+		}
+	}
+
+	return copy_struct_to_user(argp, usize, &lbm, sizeof(lbm), NULL);
+}
+
 /*
  * These long-unused ioctls were removed from the official ioctl API in 5.17,
  * but retain these definitions so that we can log warnings about them.
@@ -1467,6 +1498,9 @@ xfs_file_ioctl(
 		return xfs_ioc_verify_media(filp, arg);
 
 	default:
+		if (extensible_ioctl_valid(cmd, FS_IOC_GETLBMD_CAP,
+				LBMD_SIZE_VER0))
+			return xfs_ioc_getlbmd_cap(ip, cmd, arg);
 		return -ENOTTY;
 	}
 }