From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: [RFC][PATCH] vfs: In mntput run deactivate_super on a shallow stack. Date: Wed, 09 Apr 2014 15:58:25 -0700 Message-ID: <87fvlm860e.fsf_-_@x220.int.ebiederm.org> References: <8761v7h2pt.fsf@tw-ebiederman.twitter.com> <87li281wx6.fsf_-_@xmission.com> <87ob28kqks.fsf_-_@xmission.com> <874n3n7czm.fsf_-_@xmission.com> <87wqezl5df.fsf_-_@x220.int.ebiederm.org> <20140409023027.GX18016@ZenIV.linux.org.uk> <20140409023947.GY18016@ZenIV.linux.org.uk> <87sipmbe8x.fsf@x220.int.ebiederm.org> <20140409175322.GZ18016@ZenIV.linux.org.uk> <20140409182830.GA18016@ZenIV.linux.org.uk> <87txa286fu.fsf@x220.int.ebiederm.org> Mime-Version: 1.0 Content-Type: text/plain Cc: Linus Torvalds , "Serge E. Hallyn" , Linux-Fsdevel , Kernel Mailing List , Andy Lutomirski , Rob Landley , Miklos Szeredi , Christoph Hellwig , Karel Zak , "J. Bruce Fields" , Fengguang Wu To: Al Viro Return-path: Received: from out03.mta.xmission.com ([166.70.13.233]:51613 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932719AbaDIW6u (ORCPT ); Wed, 9 Apr 2014 18:58:50 -0400 In-Reply-To: <87txa286fu.fsf@x220.int.ebiederm.org> (Eric W. Biederman's message of "Wed, 09 Apr 2014 15:49:09 -0700") Sender: linux-fsdevel-owner@vger.kernel.org List-ID: mntput as part of pathput is called from all over the vfs sometimes as in the case of symlink chasing from some rather deep call chains. During filesystem unmount with the right set of races those innocuous little mntput calls that take very little stack space can become calls become mosters calling deactivate_super that can take up 3k or more of stack space as syncrhonous filesystem I/O is performed, through multiple levels of the I/O stack. Avoid deactivate_super being called from a deep stack by converting mntput to use task_work_add when the mnt_count goes to 0. The filesystem is still unmounted synchronously preserving the semantics that system calls like umount require. Signed-off-by: "Eric W. Biederman" --- This patch has only seen light testing so far but it emperically it appears to solve the stack depth problem. A simple umount of ext4 went from having 5162 stack bytes untouched to having 5568 stack bytes untouched. Freeing up at least 416 bytes of stack in that simple case. fs/mount.h | 2 +- fs/namespace.c | 24 ++++++++++++++++++------ 2 files changed, 19 insertions(+), 7 deletions(-) diff --git a/fs/mount.h b/fs/mount.h index aa3c0aa473df..4e78ca90467f 100644 --- a/fs/mount.h +++ b/fs/mount.h @@ -30,7 +30,7 @@ struct mount { struct mount *mnt_parent; struct dentry *mnt_mountpoint; struct vfsmount mnt; - struct rcu_head mnt_rcu; + struct callback_head mnt_callback; #ifdef CONFIG_SMP struct mnt_pcp __percpu *mnt_pcp; #else diff --git a/fs/namespace.c b/fs/namespace.c index c809205f30df..686afe9942bc 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -24,6 +24,7 @@ #include #include #include +#include #include "pnode.h" #include "internal.h" @@ -981,7 +982,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root, static void delayed_free(struct rcu_head *head) { - struct mount *mnt = container_of(head, struct mount, mnt_rcu); + struct mount *mnt = container_of(head, struct mount, mnt_callback); kfree(mnt->mnt_devname); #ifdef CONFIG_SMP free_percpu(mnt->mnt_pcp); @@ -989,6 +990,17 @@ static void delayed_free(struct rcu_head *head) kmem_cache_free(mnt_cache, mnt); } +static void mntput_delayed(struct callback_head *head) +{ + struct mount *mnt = container_of(head, struct mount, mnt_callback); + + fsnotify_vfsmount_delete(&mnt->mnt); + dput(mnt->mnt.mnt_root); + deactivate_super(mnt->mnt.mnt_sb); + mnt_free_id(mnt); + call_rcu(&mnt->mnt_callback, delayed_free); +} + static void mntput_no_expire(struct mount *mnt) { put_again: @@ -1034,11 +1046,11 @@ put_again: * so mnt_get_writers() below is safe. */ WARN_ON(mnt_get_writers(mnt)); - fsnotify_vfsmount_delete(&mnt->mnt); - dput(mnt->mnt.mnt_root); - deactivate_super(mnt->mnt.mnt_sb); - mnt_free_id(mnt); - call_rcu(&mnt->mnt_rcu, delayed_free); + /* The stack may be deep here so perform this where the stack + * is guaranteed to be shallow. + */ + init_task_work(&mnt->mnt_callback, mntput_delayed); + WARN_ON(task_work_add(current, &mnt->mnt_callback, true) != 0); } void mntput(struct vfsmount *mnt) -- 1.9.1