linux-kernel - Re: discussing about proc_misc_d

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Date:   Thu, 17 Mar 2022 12:54:21 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     hui li <juanfengpy@...il.com>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: discussing about proc_misc_d_delete

	[cc linux-kernel ]

On Mon, Mar 14, 2022 at 11:54:37AM +0800, hui li wrote:
> We noticed that, commit 1da4d377f94 (“proc: revalidate misc dentries”)
> introduced proc_misc_dentry_ops as default ops for /proc dentry,
> dentry ops for /proc/pid/net/stat/ is set as proc_net_dentry_ops,
> which will revalidate dentry each time when this path is resolved and
> dentry for the stat file is removed from dcache. This time, if files
> under /proc/pid/net/stat/ are in use, then dentries of these files
> will be put in lru when closed, which is meanlingless,  as parrent
> dentry (stat) of these files are remove from dcache.
> 
> This can be reproduced when use linux command "while :;do du
> /proc/;done”, then refcount of each dentry of /proc/pid/net/stat/ will
> increase rapidly which should be deleted at once.

Are you worried that reference count can overflow? Those dentries will be
flushed eventually and reference count goes back to normal values.
This is easy to see with "echo 3 >/proc/sys/vm/drop_caches".

> I think this problem may by solved by checking whether parrent
> dentries are in d_cache inside proc_misc_d_delete, or set
> proc_misc_dentry_ops->d_delete = always_delete_dentry, just as what is
> used in kernel version 4.x and 3.x.
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -236,6 +236,16 @@ static int proc_misc_d_revalidate(struct dentry
> *dentry, unsigned int flags)
> 
>  static int proc_misc_d_delete(const struct dentry *dentry)
>  {
> +       struct dentry *p;
> +       for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) {
> +               if (!spin_trylock(&p->d_lock))
> +                       break;
> +               if (unlikely(d_unhashed(p))){
> +                       spin_unlock(&p->d_lock);
> +                       return 1;
> +               }
> +               spin_unlock(&p->d_lock);
> +       }
>         return atomic_read(&PDE(d_inode(dentry))->in_use) < 0;
>  }