From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793209091; b=ABi/0+H2c1xckQzWObXbL2oIl2zoJkc6QmncmRAgdq9xjuo4DjYPBzOYZqugGjT4koLV 6u1m6jGX4FCOr4ZXXGZAl7/WviHWwqBukL3ze3i0DF2AEkLVtbMWfV3h6MVvrCeMO0saP dxXYz8m33A1zA4Kmqo7D60dJSbKTQREnPJU3FZyu1WqYc7Hxq6/zGJbgChKIT7yT7FcwI bKKkccF7tM21tl6+HCCg/TX3epceMs7yF6HFnkSSEAAD9rSqBy1tBq03S8BYnAJ9wZXVt vlQ1uoS+EUGPlMPkRuKA0VhU3t/CKyhsf6HZhWFaC1apgWabVXYrpYMqL6ha7PL/e3g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793209091; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:User-Agent:X-stable:X-Patchwork-Hint: MIME-Version:Content-Transfer-Encoding; bh=XWCSbzhrZVY4I/IEbjTA+YIV2/tJX34RmUTSjbSxvZA=; b=xsprFq4xo15rQuWGvY77TCb+7NiCLsWf+hBQ17jORfIB6gQCip4jq9HufoGdOHoYQmv3 YpZO4xP3Enl98c+TQ7EODOI14CI2jWFSrxbhBS+6SQ1wRhbuTgaEBxAu2JG1kXL1A1mi4 fS+j28Fz72juU2qihjDi14tIxYxY3rntOu4xi7yRkuPr2PnAHbowcuyiyCAG4Y3ZHpBdt Eip9knNB6MNrgjLewmwK9KnRXbLGsIT6TFYeRexyKe+9+07FmE/t4kMUJcR46VT+mU9kH X3rLQBpAUSxdQdNwfuOO7BNhklsJ8gdVw0fWtCR+278UBdiOPe2RJvv/VUOAdhUluZQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=tqiuAQGh; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D9B94606A324; Wed, 28 Oct 2026 17:38:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=tqiuAQGh Received: by smtp.kernel.org (Postfix) with ESMTPSA id 357021F01825; Wed, 28 Oct 2026 17:38:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1793209090; bh=XWCSbzhrZVY4I/IEbjTA+YIV2/tJX34RmUTSjbSxvZA=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=tqiuAQGhpTXzqeY54ztVq4w619FSPB6k/8G772kQ7VZHppx1SwHzpRcUAnNy5zIZX 02uHgnVUA5gHx1rRjYX7HIV4qwElmLLb4b1EC8NKjMA4ipxQutQ+Rjap18cUo7X1W5 JJNawRkr5kTgSrEJk8DKNInqBmCP+6fg9W1vDZfs= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, "Christian Brauner (Amutable)" Subject: [PATCH 5.10 349/516] namespace: check a submount for references right before unmounting it Date: Wed, 28 Oct 2026 17:58:21 +0100 Message-ID: <20261028170011.760148507@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261028170003.522232767@linuxfoundation.org> References: <20261028170003.522232767@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: patches@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 5.10-stable review patch. If anyone has any objections, please let me know. ------------------ From: Christian Brauner commit 32bf92416614a1e3ff3524fc6b780259c7aad3ef upstream. shrink_submounts() and mark_mounts_for_expiry() first collect all the mounts they are allowed to unmount and then unmount them. Whether a mount is busy is decided by propagate_mount_busy() on the tree. But unmounting the first mount changes the tree that the second one propagates into. propagate_umount() moves a surviving overmount off a copy it unmounts and mounts it where the copy was mounted. If that's where the propagated copy of the second victim is looked up the overmount becomes a candidate of the second umount_tree(). It's childless and so trim_one() commits it without looking at its reference count. So a synchronous umount pulls out a busy mount that is in use somewhere else and marks it MNT_SYNC_UMOUNT while it has users: umount2(/tmp/plshrink/p1, 0) = 0 errno 0 cwd is (unreachable) The same two umounts requested one after the other fail with EBUSY. This needs a mount with MNT_SHRINKABLE, i.e., automounted submounts of NFS, AFS and CIFS or the tracefs mount below debugfs. Bind mounts inherit the flag. Check each mount right before it is unmounted under the same hold of namespace_sem and mount_lock as the umount itself. Make sure that the algorithm stays linear. Fixes: 1064f874abc0 ("mnt: Tuck mounts under others instead of creating shadow/side mounts.") Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-2-be34c83956ae@kernel.org Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman --- fs/namespace.c | 78 ++++++++++++++++++++++++++++++++++++++------------------- 1 file changed, 53 insertions(+), 25 deletions(-) --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3126,6 +3126,11 @@ void mark_mounts_for_expiry(struct list_ } while (!list_empty(&graveyard)) { mnt = list_first_entry(&graveyard, struct mount, mnt_expire); + /* an earlier umount_tree() may have moved a busy mount here */ + if (propagate_mount_busy(mnt, 1)) { + list_move(&mnt->mnt_expire, mounts); + continue; + } touch_mnt_namespace(mnt->mnt_ns); umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC); } @@ -3136,16 +3141,36 @@ void mark_mounts_for_expiry(struct list_ EXPORT_SYMBOL_GPL(mark_mounts_for_expiry); /* + * Unmount @mnt if it's a shrinkable mount without children that nobody uses. + * + * mount_lock must be held for write + */ +static bool shrink_submount(struct mount *mnt) +{ + if (propagate_mount_busy(mnt, 1)) + return false; + touch_mnt_namespace(mnt->mnt_ns); + umount_tree(mnt, UMOUNT_PROPAGATE|UMOUNT_SYNC); + return true; +} + +/* * Ripoff of 'select_parent()' * - * search the list of submounts for a given mountpoint, and move any - * shrinkable submounts to the 'graveyard' list. + * unmount the shrinkable submounts of @parent that aren't busy, children + * before their parent, and say whether anything went + * + * The cursor into the children of @this_parent survives the umount of a + * child mount without child mounts. The mounts that get umounted together with + * it are located under receiving mounts of @this_parent and never under + * @this_parent itself. The one exception is @this_parent getting unmounted + * then the walk starts over. */ -static int select_submounts(struct mount *parent, struct list_head *graveyard) +static bool __shrink_submounts(struct mount *parent) { struct mount *this_parent = parent; struct list_head *next; - int found = 0; + bool shrunk = false; repeat: next = this_parent->mnt_mounts.next; @@ -3164,42 +3189,45 @@ resume: this_parent = mnt; goto repeat; } - - if (!propagate_mount_busy(mnt, 1)) { - list_move_tail(&mnt->mnt_expire, graveyard); - found++; - } + if (!shrink_submount(mnt)) + continue; + shrunk = true; + if (unlikely(this_parent->mnt.mnt_flags & MNT_UMOUNT)) + return true; } /* * All done at this level ... ascend and resume the search */ if (this_parent != parent) { - next = this_parent->mnt_child.next; - this_parent = this_parent->mnt_parent; + struct mount *mnt = this_parent; + + next = mnt->mnt_child.next; + this_parent = mnt->mnt_parent; + /* its children are gone, maybe it can go as well */ + if (shrink_submount(mnt)) { + shrunk = true; + if (unlikely(this_parent->mnt.mnt_flags & MNT_UMOUNT)) + return true; + } goto resume; } - return found; + return shrunk; } /* - * process a list of expirable mountpoints with the intent of discarding any - * submounts of a specific parent mountpoint + * unmount the shrinkable submounts of @mnt that aren't busy + * + * The busy check and the umount of a mount are adjacent. An umount can + * still empty or move a mount in a part of the tree that was walked + * already, so walk again until nothing goes. * * mount_lock must be held for write */ static void shrink_submounts(struct mount *mnt) { - LIST_HEAD(graveyard); - struct mount *m; - - /* extract submounts of 'mountpoint' from the expiration list */ - while (select_submounts(mnt, &graveyard)) { - while (!list_empty(&graveyard)) { - m = list_first_entry(&graveyard, struct mount, - mnt_expire); - touch_mnt_namespace(m->mnt_ns); - umount_tree(m, UMOUNT_PROPAGATE|UMOUNT_SYNC); - } + for (;;) { + if (!__shrink_submounts(mnt)) + break; } }