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

[Joe Perches]

On Mon, 2016-08-22 at 09:24 +0200, Michal Hocko wrote:
> 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.

I completely disagree here with your rationale here.

I think you didn't read the code and didn't try it either.

This code is identical to the previous code but it
simply estimates the required output size first.

> 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.

It's never been like that as far as I know.

Please read fs/seq_file.c:traverse()

This code starts with a PAGE_SIZE block of memory then if
the complete output doesn't fit, stops, frees that block
of memory, and retries the complete output with a last block
size allocated << 1 and tries again.

> 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.

I've tried it, I wish you would.