Patch "mnt: In propgate_umount handle visiting mounts in any order" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

mnt: In propgate_umount handle visiting mounts in any order

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mnt-in-propgate_umount-handle-visiting-mounts-in-any-order.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Eric W. Biederman" <[email protected]>
Date: Mon, 24 Oct 2016 16:16:13 -0500
Subject: mnt: In propgate_umount handle visiting mounts in any order

From: Eric W. Biederman <[email protected]>

commit 99b19d16471e9c3faa85cad38abc9cbbe04c6d55 upstream.

While investigating some poor umount performance I realized that in the case of overlapping mount trees where some of the mounts are locked the code has been failing to unmount all of the mounts it should have been unmounting.

This failure to unmount all of the necessary
mounts can be reproduced with:

$ cat locked_mounts_test.sh

mount -t tmpfs test-base /mnt
mount --make-shared /mnt
mkdir -p /mnt/b

mount -t tmpfs test1 /mnt/b
mount --make-shared /mnt/b
mkdir -p /mnt/b/10

mount -t tmpfs test2 /mnt/b/10
mount --make-shared /mnt/b/10
mkdir -p /mnt/b/10/20

mount --rbind /mnt/b /mnt/b/10/20

unshare -Urm --propagation unchaged /bin/sh -c 'sleep 5; if [ $(grep test /proc/self/mountinfo | wc -l) -eq 1 ] ; then echo SUCCESS ; else echo FAILURE ; fi' sleep 1
umount -l /mnt/b
wait %%

$ unshare -Urm ./locked_mounts_test.sh

This failure is corrected by removing the prepass that marks mounts that may be umounted.

A first pass is added that umounts mounts if possible and if not sets mount mark if they could be unmounted if they weren't locked and adds them to a list to umount possibilities. This first pass reconsiders the mounts parent if it is on the list of umount possibilities, ensuring that information of umoutability will pass from child to mount parent.

A second pass then walks through all mounts that are umounted and processes their children unmounting them or marking them for reparenting.

A last pass cleans up the state on the mounts that could not be umounted and if applicable reparents them to their first parent that remained mounted.

While a bit longer than the old code this code is much more robust as it allows information to flow up from the leaves and down from the trunk making the order in which mounts are encountered in the umount propgation tree irrelevant.

---
 fs/mount.h     |    2 
 fs/namespace.c |    2 
 fs/pnode.c     |  150 +++++++++++++++++++++++++++++++++------------------------
 3 files changed, 91 insertions(+), 63 deletions(-)

@@ -58,7 +58,7 @@ struct mount {
 	struct mnt_namespace *mnt_ns;	/* containing namespace */
 	struct mountpoint *mnt_mp;	/* where is it mounted */
 	struct hlist_node mnt_mp_list;	/* list mounts with the same mountpoint */
-	struct list_head mnt_reparent;	/* reparent list entry */
+	struct list_head mnt_umounting; /* list entry for umount propagation */
 #ifdef CONFIG_FSNOTIFY
 	struct hlist_head mnt_fsnotify_marks;
 	__u32 mnt_fsnotify_mask;

@@ -236,7 +236,7 @@ static struct mount *alloc_vfsmnt(const
 		INIT_LIST_HEAD(&mnt->mnt_slave_list);
 		INIT_LIST_HEAD(&mnt->mnt_slave);
 		INIT_HLIST_NODE(&mnt->mnt_mp_list);
-		INIT_LIST_HEAD(&mnt->mnt_reparent);
+		INIT_LIST_HEAD(&mnt->mnt_umounting);
 #ifdef CONFIG_FSNOTIFY
 		INIT_HLIST_HEAD(&mnt->mnt_fsnotify_marks);
 #endif

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/mnt-in-propgate_umount-handle-visiting-mounts-in-any-order.patch queue-4.11/selftests-capabilities-fix-the-test_execve-test.patch queue-4.11/mnt-in-umount-propagation-reparent-in-a-separate-pass.patch queue-4.11/mnt-make-propagate_umount-less-slow-for-overlapping-mount-propagation-trees.patch