Re: [RFC][PATCH 7/14] Union-mount mounting

Jan Engelhardt
2007-05-15T07:31:25Z

On May 14 2007 15:11, Bharata B Rao wrote:
>
>TODO: bind and move mounts aren't yet supported with union mounts.

Are the semantics already set?

>@@ -294,6 +294,10 @@ static struct vfsmount *clone_mnt(struct
> if (!mnt)
> goto alloc_failed;
>
>+ /*
>+ * As of now, cloning of union mounted mnt isn't permitted.
>+ */
>+ BUG_ON(mnt->mnt_flags & MNT_UNION);

One, please avoid BUG_ONs. Now I am not sure if clone_mnt is called
as part of kthread creation when CLONE_FS is it. If so, get rid of
this one real fast. Also see chunk "@@ -1031,.. @@ do_loopback"
below.

if(mnt->mnt_flags & MNT_UNION)
goto return_einval;

or something.

>+#ifdef CONFIG_UNION_MOUNT
>+ struct union_info *uinfo = NULL;
>+#endif
>
> retval = security_sb_umount(mnt, flags);
> if (retval)
>@@ -685,6 +696,14 @@ static int do_umount(struct vfsmount *mn
> }
>
> down_write(&namespace_sem);
>+#ifdef CONFIG_UNION_MOUNT
>+ /*
>+ * Grab a reference to the union_info which gets detached
>+ * from the dentries in release_mounts().
>+ */
>+ if (mnt->mnt_flags & MNT_UNION)
>+ uinfo = union_lock_and_get(mnt->mnt_root);
>+#endif
> spin_lock(&vfsmount_lock);
> event++;
>
>@@ -699,6 +718,15 @@ static int do_umount(struct vfsmount *mn
> security_sb_umount_busy(mnt);
> up_write(&namespace_sem);
> release_mounts(&umount_list);
>+#ifdef CONFIG_UNION_MOUNT
>+ if (uinfo) {
>+ if (atomic_read(&uinfo->u_count) == 1)
>+ /* We are the last user of this union_info */
>+ union_release(uinfo);
>+ else
>+ union_put_and_unlock(uinfo);
>+ }
>+#endif
> return retval;
> }
>

Is it feasible to do with with some less #if/#endif magic?:

>@@ -1031,6 +1070,15 @@ static int do_loopback(struct nameidata
> if (err)
> return err;
>
>+ /*
>+ * bind mounting to or from union mounts is not supported
>+ */
>+ err = -EINVAL;
>+ if (nd->mnt->mnt_flags & MNT_UNION)
>+ goto out_unlocked;
>+ if (old_nd.mnt->mnt_flags & MNT_UNION)
>+ goto out_unlocked;
>+

Do the same in clone_mnt.

> down_write(&namespace_sem);
> err = -EINVAL;
> if (IS_MNT_UNBINDABLE(old_nd.mnt))
>@@ -1064,6 +1112,7 @@ static int do_loopback(struct nameidata
>
> out:
> up_write(&namespace_sem);
>+out_unlocked:
> path_release(&old_nd);
> return err;
> }

>+++ b/include/linux/fs.h
>@@ -1984,6 +1984,9 @@ static inline ino_t parent_ino(struct de
> /* kernel/fork.c */
> extern int unshare_files(void);
>
>+/* fs/union.c */
>+#include <linux/union.h>
>+
> /* Transaction based IO helpers */
>
> /*

This raises a big eyebrow. If linux/fs.h can compile without the
inclusion of linux/union.h, do not put linux/union.h in fs.h.

>+#ifdef CONFIG_UNION_MOUNT
>+
>+#include <linux/fs_struct.h>
>+
>+/* namespace stuff used at mount time */
>+extern void attach_mnt_union(struct vfsmount *, struct nameidata *);
>+extern void detach_mnt_union(struct vfsmount *, struct path *);

You do not need that #include I suppose. Just predeclare the structs.

struct path;
struct vfsmount;
extern void ...

Saves us the "compiler slurps in so many .h files" problem.

Jan
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