linux-kernel - Re: + mm-proc-pid-smaps_rollup-convert-to-single-value-seq

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Message-ID: <20180724182908.GD27053@avx2>
Date:   Tue, 24 Jul 2018 21:29:08 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, dancol@...gle.com, vbabka@...e.cz
Subject: Re: +
 mm-proc-pid-smaps_rollup-convert-to-single-value-seq_file.patch added to -mm
 tree

On Mon, Jul 23, 2018 at 04:55:48PM -0700, akpm@...ux-foundation.org wrote:
> The patch titled
>      Subject: mm: /proc/pid/smaps_rollup: convert to single value seq_file
> has been added to the -mm tree.  Its filename is
>      mm-proc-pid-smaps_rollup-convert-to-single-value-seq_file.patch

> Subject: mm: /proc/pid/smaps_rollup: convert to single value seq_file
> 
> The /proc/pid/smaps_rollup file is currently implemented via the
> m_start/m_next/m_stop seq_file iterators shared with the other maps files,
> that iterate over vma's.  However, the rollup file doesn't print anything
> for each vma, only accumulate the stats.

What I don't understand why keep seq_ops then and not do all the work in
->show hook.  Currently /proc/*/smaps_rollup is at ~500 bytes so with
minimum 1 page seq buffer, no buffer resizing is possible.

> +static int show_smaps_rollup(struct seq_file *m, void *v)
> +{
> +	struct proc_maps_private *priv = m->private;
> +	struct mem_size_stats *mss = priv->rollup;
> +	struct vm_area_struct *vma;
> +
> +	/*
> +	 * We might be called multiple times when e.g. the seq buffer
> +	 * overflows. Gather the stats only once.

It doesn't!

> +	if (!mss->finished) {
> +		for (vma = priv->mm->mmap; vma; vma = vma->vm_next) {
> +			smap_gather_stats(vma, mss);
> +			mss->last_vma_end = vma->vm_end;
>  		}
> -		last_vma = !m_next_vma(priv, vma);
> -	} else {
> -		rollup_mode = false;
> -		memset(&mss_stack, 0, sizeof(mss_stack));
> -		mss = &mss_stack;