linux-kernel - Re: [PATCH 2/2] proc: task_mmu: Reduce output processing cpu time

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Message-ID: <20160822072414.GB13596@dhcp22.suse.cz>
Date:   Mon, 22 Aug 2016 09:24:15 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joe Perches <joe@...ches.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jann@...jh.net>, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] proc: task_mmu: Reduce output processing cpu time

On Sat 20-08-16 01:00:17, Joe Perches wrote:
[...]
>  static int proc_maps_open(struct inode *inode, struct file *file,
>  			const struct seq_operations *ops, int psize)
>  {
> -	struct proc_maps_private *priv = __seq_open_private(file, ops, psize);
> +	struct proc_maps_private *priv;
> +	struct mm_struct *mm;
> +
> +	mm = proc_mem_open(inode, PTRACE_MODE_READ);
> +	if (IS_ERR(mm))
> +		return PTR_ERR(mm);
>  
> +	priv = __seq_open_private_bufsize(file, ops, psize,
> +					  mm && mm->map_count ?
> +					  mm->map_count * 0x300 : PAGE_SIZE);

NAK to this! Seriously, this just gives any random user access to user
defined amount of memory which not accounted, not reclaimable and a
potential consumer of any higher order blocks.

Besides that, at least one show_smap output will always fit inside the
single page and AFAIR (it's been quite a while since I've looked into
seq_file internals) the buffer grows only when the single show doesn't
fit in.

>  	if (!priv)
>  		return -ENOMEM;
>  
>  	priv->inode = inode;
> -	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
> -	if (IS_ERR(priv->mm)) {
> -		int err = PTR_ERR(priv->mm);
> -
> -		seq_release_private(inode, file);
> -		return err;
> -	}
> +	priv->mm = mm;
>  
>  	return 0;
>  }
> @@ -721,6 +723,25 @@ void __weak arch_show_smap(struct seq_file *m, struct vm_area_struct *vma)
>  {
>  }
>  
> +static void show_val_kb(struct seq_file *m, const char *s, unsigned long num)
> +{
> +	char v[32];
> +	static const char blanks[7] = {' ', ' ', ' ', ' ',' ', ' ', ' '};
> +	int len;
> +
> +	len = num_to_str(v, sizeof(v), num >> 10);
> +
> +	seq_write(m, s, 16);
> +
> +	if (len > 0) {
> +		if (len < 8)
> +			seq_write(m, blanks, 8 - len);
> +
> +		seq_write(m, v, len);
> +	}
> +	seq_write(m, " kB\n", 4);
> +}
> +

I really do not understand why you insist on code duplication rather
than reuse but if you really insist then just make this (without the
above __seq_open_private_bufsize, re-measure and add the results to the
changelog and repost.
-- 
Michal Hocko
SUSE Labs