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

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