linux-kernel - Re: About mounting the sysfs

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Date:	Tue, 18 Dec 2007 23:05:48 +0800
From:	Zhanhua <is01kzh@...il.com>
To:	"Jan Engelhardt" <jengelh@...putergmbh.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: About mounting the sysfs

2007/12/18, Jan Engelhardt <jengelh@...putergmbh.de>:
>
> On Dec 18 2007 15:10, wit wrote:
> >
> >1. What is the d_alloc_root used for? Actually, the question should
> >be: why we have to call d_alloc_root.
>
> >I think the root already has its dentry,
>
> It does not.
There's no dentry for the "/"? I mean the rootfs.

>
> >why we have to allocate another while we mounting a file
> >system?
> >
> >2. Why we call d_alloc_root to allocate a dentry for the mount point
> >while the usual mount point of sysfs is defined by the user (something
> >like /sysfs but not /).
>
> /sys is a dentry that belongs to the / vfsmount, but we need a
> / that belongs to the <whatever you are going to mount> vfsmount.

Why we need such a vfsmount (for the "/", not the rootfs)? And where
we store the mount point info (path) when mount_root, s_root and the
mnt_mountpoint are all points to the "/" which is allocated by
d_alloc_root? Or do we have to store such info? Why?

>
> > See below:
> >       root = d_alloc_root(inode);
> >       if (!root) {
> >               pr_debug("%s: could not get root dentry!\n",__FUNCTION__);
> >               iput(inode);
> >               return -ENOMEM;
> >       }
> >       root->d_fsdata = &sysfs_root;
> >       sb->s_root = root;
> >
> >does this means settting the sysfs' mount point to "/" but not "/sysfs".
>
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