Re: [PATCH RFC] mm: account VMA before forced-COW via/proc/pid/mem

[Hugh Dickins]

On Thu, 5 Apr 2012, Konstantin Khlebnikov wrote:
> Oleg Nesterov wrote:
> > On 04/04, Konstantin Khlebnikov wrote:
> > > Oleg Nesterov wrote:
> > > > On 04/02, Konstantin Khlebnikov wrote:
> > > > > 
> > > > > Currently kernel does not account read-only private mappings into
> > > > > memory commitment.
> > > > > But these mappings can be force-COW-ed in get_user_pages().
> > > > 
> > > > Heh. tail -n3 Documentation/vm/overcommit-accounting
> > > > may be you should update it then.
> > > 
> > > I just wonder how fragile this accounting...
> > 
> > I meant, this patch could also remove this "TODO" from the docs.
> 
> Actually I dug into this code for killing VM_ACCOUNT vma flag.
> Currently we cannot do this only because asymmetry in mprotect_fixup():
> it account vma on read-only -> writable conversion, but keep on backward
> operation.
> Probably we can kill this asymmetry, and after that we can recognize
> accountable vma
> by its others flags state, so we don't need special VM_ACCOUNT for this.

(I believe the VM_ACCOUNT flag will need to stay.)

But this is just a quick note to say that I'm not ignoring you: I have
a strong interest in this, but only now found time to look through the
thread and ponder, and I'm not yet ready to decide.

I've long detested that behaviour of GUP write,force, and my strong
preference would be not to layer more strangeness upon strangeness,
but limit the damage by making GUP write,force fail in that case,
instead of inserting a PageAnon page into a VM_SHARED mapping.

I think it's unlikely that it will cause a regression in real life
(it already fails if you did not open the mmap'ed file for writing),
but it would be a user-visible change in behaviour, and I've research
to do before arriving at a conclusion.

Hugh
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