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fix the softlockups in attach_recursive_mnt()

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In case when we mounting something on top of a large stack of overmounts,
all of them being peers of each other, we get quadratic time by the
depth of overmount stack.  Easily fixed by doing commit_tree() before
reparenting the overmount; simplifies commit_tree() as well - it doesn't
need to skip the already mounted stuff that had been reparented on top
of the new mounts.

Since we are holding mount_lock through both reparenting and call of
commit_tree(), the order does not matter from the mount hash point
of view.

Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Tested-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Fixes: 663206854f "copy_tree(): don't link the mounts via mnt_list"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2025-08-15 15:38:14 -04:00
parent 8742b2d893
commit 0ddfb62f5d
1 changed files with 2 additions and 5 deletions
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7
fs/namespace.c
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@ -1197,10 +1197,7 @@ static void commit_tree(struct mount *mnt)
if (!mnt_ns_attached(mnt)) {
for (struct mount *m = mnt; m; m = next_mnt(m, mnt))
if (unlikely(mnt_ns_attached(m)))
m = skip_mnt_tree(m);
else
mnt_add_to_ns(n, m);
mnt_add_to_ns(n, m);
n->nr_mounts += n->pending_mounts;
n->pending_mounts = 0;
}
@ -2704,6 +2701,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
lock_mnt_tree(child);
q = __lookup_mnt(&child->mnt_parent->mnt,
child->mnt_mountpoint);
commit_tree(child);
if (q) {
struct mountpoint *mp = root.mp;
struct mount *r = child;
@ -2713,7 +2711,6 @@ static int attach_recursive_mnt(struct mount *source_mnt,
mp = shorter;
mnt_change_mountpoint(r, mp, q);
}
commit_tree(child);
}
unpin_mountpoint(&root);
unlock_mount_hash();