Re: [PATCH v3 1/2] mm, proc: Fix region lost in /proc/self/smaps

[Robert Hu <robert.hu@vmm.sh.intel.com>]

On Mon, 2016-09-26 at 10:46 +0200, Michal Hocko wrote:
> On Fri 23-09-16 17:53:51, Oleg Nesterov wrote:
> > On 09/23, Michal Hocko wrote:
> > >
> > > On Fri 23-09-16 15:56:36, Oleg Nesterov wrote:
> > > > 
> > > > I think we can simplify this patch. And imo make it better. How about
> > > 
> > > it is certainly less subtle because it doesn't report "sub-vmas".
> > > 
> > > > 	if (last_addr) {
> > > > 		vma = find_vma(mm, last_addr - 1);
> > > > 		if (vma && vma->vm_start <= last_addr)
> > > > 			vma = m_next_vma(priv, vma);
> > > > 		if (vma)
> > > > 			return vma;
> > > > 	}
> > > 
> > > we would still miss a VMA if the last one got shrunk/split
> > 
> > Not sure I understand what you mean... If the last one was split
> > we probably should not report the new vma.
> 
> Right, VMA split is less of a problem. I meant to say that if the
> last_vma->vm_end got lower for whatever reason then we could miss a VMA
> right after. We actually might want to display such a VMA because it
> could be a completely new one. We just do not know whether it is a
> former split with enlarged VMA or a completely new one
> 
> [      old VMA     ]   Hole       [   VMA    ]
> [ old VMA   ][  New VMa    ]      [   VMA    ]

This is indeed possible. But I see this is like the last_vma enlargement
case. I suggest we accept such missing, as we accept the enlargement
part of last_vma is not printed.

How about we set such target: 1) no duplicate print; 2) no old vma
missing (unless it's unmapped); 3) monotonic printing.
We accept those newly added/changed parts between 2 partial reads is not
printed.

How about above suggestion? If you, Dave, Oleg and others accept it,
then Oleg's improvement can achieve it, I think.
> 
> > Nevermind, in any case yes, sure, this can't "fix" other corner cases.
> 
> Agreed, or at least I do not see an easy way for that.


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