KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 63 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/linear.rst b/Documentation/admin-guide/device-mapper/linear.rst
new file mode 100644
index 000000000..9d17fc6e6
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/linear.rst
@@ -0,0 +1,63 @@
+=========
+dm-linear
+=========
+
+Device-Mapper's "linear" target maps a linear range of the Device-Mapper
+device onto a linear range of another device.  This is the basic building
+block of logical volume managers.
+
+Parameters: <dev path> <offset>
+    <dev path>:
+	Full pathname to the underlying block-device, or a
+        "major:minor" device-number.
+    <offset>:
+	Starting sector within the device.
+
+
+Example scripts
+===============
+
+::
+
+  #!/bin/sh
+  # Create an identity mapping for a device
+  echo "0 `blockdev --getsz $1` linear $1 0" | dmsetup create identity
+
+::
+
+  #!/bin/sh
+  # Join 2 devices together
+  size1=`blockdev --getsz $1`
+  size2=`blockdev --getsz $2`
+  echo "0 $size1 linear $1 0
+  $size1 $size2 linear $2 0" | dmsetup create joined
+
+::
+
+  #!/usr/bin/perl -w
+  # Split a device into 4M chunks and then join them together in reverse order.
+
+  my $name = "reverse";
+  my $extent_size = 4 * 1024 * 2;
+  my $dev = $ARGV[0];
+  my $table = "";
+  my $count = 0;
+
+  if (!defined($dev)) {
+          die("Please specify a device.\n");
+  }
+
+  my $dev_size = `blockdev --getsz $dev`;
+  my $extents = int($dev_size / $extent_size) -
+                (($dev_size % $extent_size) ? 1 : 0);
+
+  while ($extents > 0) {
+          my $this_start = $count * $extent_size;
+          $extents--;
+          $count++;
+          my $this_offset = $extents * $extent_size;
+
+          $table .= "$this_start $extent_size linear $dev $this_offset\n";
+  }
+
+  `echo \"$table\" | dmsetup create $name`;