[Xen-devel] [PATCH 1/1] tools/hotplug: Scan xenstore once when attaching shared images files

[Mike Latimer <mlatimer@xxxxxxxx>]

Create the list of shared loopback devices from within check_sharing, rather
than calling check_sharing for every loopback device using the same shared
image.

This change prevents the xenstore database from being walked for every shared
device, which causes an exponential decrease in performance. 

Signed-off-by: Mike Latimer <mlatimer@xxxxxxxx>
---
 tools/hotplug/Linux/block | 67 +++++++++++++++++++++++++++++------------------
 1 file changed, 41 insertions(+), 26 deletions(-)

diff --git a/tools/hotplug/Linux/block b/tools/hotplug/Linux/block
index 8d2ee9d..aef051c 100644
--- a/tools/hotplug/Linux/block
+++ b/tools/hotplug/Linux/block
@@ -38,7 +38,7 @@ find_free_loopback_dev() {
 }
 
 ##
-# check_sharing device mode
+# check_sharing devtype device mode [inode]
 #
 # Check whether the device requested is already in use.  To use the device in
 # read-only mode, it may be in use in read-only mode, but may not be in use in
@@ -56,10 +56,27 @@ find_free_loopback_dev() {
 #
 check_sharing()
 {
-  local dev="$1"
-  local mode="$2"
+  local devtype=$1
+  local dev="$2"
+  local mode="$3"
+
+  if [ "$devtype" = "file" ];
+  then
+    local inode="$4"
+
+    shared_list=$(losetup -a |
+          sed -n -e 
"s@^\([^:]\+\)\(:[[:blank:]]\[0*${dev}\]:${inode}[[:blank:]](.*)\)@\1@p" )
+    for dev in $shared_list
+    do
+      if [ -n "$dev" ]
+      then
+        devmm="${devmm}$(device_major_minor $dev),"
+      fi
+    done
+  else
+    local devmm="$(device_major_minor "$dev"),"
+  fi
 
-  local devmm=$(device_major_minor "$dev")
   local file
 
   if [ "$mode" = 'w' ]
@@ -75,9 +92,9 @@ check_sharing()
     then
       local d=$(device_major_minor "$file")
 
-      if [ "$d" = "$devmm" ]
+      if [[ "$devmm" == *"$d,"* ]]
       then
-        echo 'local'
+        echo "local $d"
         return
       fi
     fi
@@ -90,13 +107,13 @@ check_sharing()
     do
       d=$(xenstore_read_default "$base_path/$dom/$dev/physical-device" "")
 
-      if [ "$d" = "$devmm" ]
+      if [ -n "$d" ] && [[ "$devmm" == *"$d,"* ]]
       then
         if [ "$mode" = 'w' ]
         then
           if ! same_vm $dom
           then
-            echo 'guest'
+            echo "guest $d"
             return
           fi
         else
@@ -107,7 +124,7 @@ check_sharing()
           then
             if ! same_vm $dom
             then
-              echo 'guest'
+              echo "guest $d"
               return
             fi
           fi
@@ -129,6 +146,7 @@ check_device_sharing()
 {
   local dev="$1"
   local mode=$(canonicalise_mode "$2")
+  local type="device"
   local result
 
   if [ "x$mode" = 'x!' ]
@@ -136,33 +154,38 @@ check_device_sharing()
     return 0
   fi
 
-  result=$(check_sharing "$dev" "$mode")
+  result=$(check_sharing "$type" "$dev" "$mode")
 
   if [ "$result" != 'ok' ]
   then
-    do_ebusy "Device $dev is mounted " "$mode" "$result"
+    do_ebusy "Device $dev is mounted " "$mode" "${result%% *}"
   fi
 }
 
 
 ##
-# check_device_sharing file dev mode
+# check_device_sharing file dev mode inode
 #
-# Perform the sharing check for the given file mounted through the given
-# loopback interface, in the given mode.
+# Perform the sharing check for the given file, with its corresponding
+# device, inode and mode. As the file can be mounted multiple times,
+# the inode is passed through to check_sharing for all instances to be
+# checked.
 #
 check_file_sharing()
 {
   local file="$1"
   local dev="$2"
   local mode="$3"
+  local inode="$4"
+  local type="file"
+  local result
 
-  result=$(check_sharing "$dev" "$mode")
+  result=$(check_sharing "$type" "$dev" "$mode" "$inode")
 
   if [ "$result" != 'ok' ]
   then
-    do_ebusy "File $file is loopback-mounted through $dev,
-which is mounted " "$mode" "$result"
+    do_ebusy "File $file is loopback-mounted through ${result#* },
+which is mounted " "$mode" "${result%% *}"
   fi
 }
 
@@ -281,15 +304,7 @@ mount it read-write in a guest domain."
             fatal "Unable to lookup $file: dev: $dev inode: $inode"
           fi
 
-          shared_list=$(losetup -a |
-                sed -n -e 
"s@^\([^:]\+\)\(:[[:blank:]]\[0*${dev}\]:${inode}[[:blank:]](.*)\)@\1@p" )
-          for dev in $shared_list
-          do
-            if [ -n "$dev" ]
-            then
-              check_file_sharing "$file" "$dev" "$mode"
-            fi
-          done
+          check_file_sharing "$file" "$dev" "$mode" "$inode"
         fi
 
         loopdev=$(losetup -f 2>/dev/null || find_free_loopback_dev)
-- 
1.8.4.5


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