Re: [PATCH 4/4] mm: proc/pid/smaps_rollup: convert to single value seq_file

[Alexey Dobriyan <adobriyan@gmail.com>]

On Wed, Jul 25, 2018 at 08:53:53AM +0200, Vlastimil Babka wrote:
> I moved the reply to this thread since the "added to -mm tree"
> notification Alexey replied to in <20180724182908.GD27053@avx2> has
> reduced CC list and is not linked to the patch postings.
> 
> On 07/24/2018 08:29 PM, Alexey Dobriyan wrote:
> > On Mon, Jul 23, 2018 at 04:55:48PM -0700, akpm@linux-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.
> 
> Hmm IIUC seq_file also provides the buffer and handles feeding the data
> from there to the user process, which might have called read() with a smaller
> buffer than that. So I would rather not avoid the seq_file infrastructure.
> Or you're saying it could be converted to single_open()? Maybe, with more work.

Prefereably yes.

There are 2 ways to using seq_file:
* introduce seq_operations and iterate over objects printing them one by one,
* use single_open and 1 ->show hook and do all the work of collecting
  data there and print once.

  /proc/*/smaps_rollup is suited for variant 2 because variant 1 is
  designed for printing arbitrary amount of data.


> >> +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!
> 
> Because the buffer is 1 page and the data is ~500 bytes as you said above?
> Agreed, but I wouldn't want to depend on data not growing in the future or
> the initial buffer not getting smaller. I could extend the comment that this
> is theoretical for now?

Given the rate of growth I wouldn't be concerned.

> >> +	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;