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    Subject[PATCH 34/73] union-mount: Create build_root_union() [ver #2]
    Date
    From: Valerie Aurora <vaurora@redhat.com>

    During mount(), build_root_union() creates the union stack for the
    root directory. All other directory union stacks are bootstrapped
    from their parents' union stacks during path lookup.

    Original-author: Valerie Aurora <vaurora@redhat.com>
    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    fs/namespace.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 49 insertions(+), 0 deletions(-)

    diff --git a/fs/namespace.c b/fs/namespace.c
    index 1f24a6b..3355b99 100644
    --- a/fs/namespace.c
    +++ b/fs/namespace.c
    @@ -22,6 +22,7 @@
    #include <linux/uaccess.h>
    #include "pnode.h"
    #include "internal.h"
    +#include "union.h"

    #define HASH_SHIFT ilog2(PAGE_SIZE / sizeof(struct list_head))
    #define HASH_SIZE (1UL << HASH_SHIFT)
    @@ -1520,6 +1521,54 @@ static int clone_union_tree(struct mount *topmost, struct path *mntpnt)
    return 0;
    }

    +/**
    + * build_root_union - Create the union stack for the root dir
    + * @topmost_mnt - vfsmount of topmost mount
    + *
    + * Build the union stack for the root dir. Annoyingly, we have to traverse
    + * union "up" from the root of the cloned tree to find the topmost read-only
    + * mount, and then traverse back "down" to build the stack.
    + */
    +static int build_root_union(struct vfsmount *topmost_mnt)
    +{
    + struct path lower, topmost_path;
    + struct mount *mnt, *topmost_ro_mnt;
    + unsigned int i, layers = 1;
    + int err = 0;
    +
    + /* Find the topmost read-only mount */
    + topmost_ro_mnt = real_mount(topmost_mnt->mnt_sb->s_union_lower_mnts);
    + for (mnt = topmost_ro_mnt; mnt; mnt = next_mnt(mnt, topmost_ro_mnt)) {
    + if (mnt->mnt_parent == topmost_ro_mnt &&
    + mnt->mnt_mountpoint == topmost_ro_mnt->mnt.mnt_root) {
    + topmost_ro_mnt = mnt;
    + layers++;
    + }
    + }
    + topmost_mnt->mnt_sb->s_union_count = layers;
    +
    + // SHOULD USE collect_mounts() here rather than merely mntgetting
    +
    + /* Build the root dir's union stack from the top down */
    + topmost_path.mnt = topmost_mnt;
    + topmost_path.dentry = topmost_mnt->mnt_root;
    + mnt = topmost_ro_mnt;
    + for (i = 0; i < layers; i++) {
    + lower.mnt = mntget(&mnt->mnt); // !!!!!!!!!! TODO: FIX
    + lower.dentry = dget(mnt->mnt.mnt_root);
    + err = union_add_dir(&topmost_path, &lower, i);
    + if (err)
    + goto out;
    + mnt = mnt->mnt_parent;
    + }
    + return 0;
    +
    +out:
    + d_free_unions(topmost_path.dentry);
    + topmost_mnt->mnt_sb->s_union_count = 0;
    + return err;
    +}
    +
    /*
    * @source_mnt : mount tree to be attached
    * @nd : place the mount tree @source_mnt is attached


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