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SubjectRe: [PATCH] include private data mappings in RLIMIT_DATA limit
On Tuesday 10 July 2007, Hugh Dickins wrote:
> On Tue, 10 Jul 2007, Dave McCracken wrote:
> > Given that RLIMIT_DATA is pretty much meaningless in current kernels, I
> > would put forward the argument that this change is extremely unlikely to
> > break anything because no one is currently setting it to anything other
> > than unlimited. Adding this feature would give administrators another
> > tool, a way to control the private data size of a process without
> > restricting its ability to attach to large shared mappings.
> That may be a good argument (though "extremely unlikely to break"s
> have a nasty habit of biting). I'd still say that the contribution
> to Committed_AS is more appropriate and more useful here.

You may be right... I suppose everything will bite someone somewhere with a
sufficiently large user base.

As for whether Committed_AS is more appropriate, I'll have to defer to Herbert
on this one. He stated that RLIMIT_DATA no longer does what it was intended
to do, and offered a fix for it, and I agreed with him. I do believe his
patch does a reasonable approximation of the original intent of RLIMIT_DATA,
but I didn't delve into the actual intended use of it once it's fixed.

> > > That change to /proc/PID/status VmData:
> > > - data = mm->total_vm - mm->shared_vm - mm->stack_vm;
> > > + data = mm->total_vm - mm->shared_vm - mm->stack_vm - mm->exec_vm;
> > > looks plausible, but isn't exec_vm already counted as shared_vm,
> > > so now being doubly subtracted? Besides which, we wouldn't want
> > > to change those numbers again without consulting Albert.
> >
> > As I recall, this was added after Herbert discovered that exec_vm is not
> > counted as shared_vm. It's actually mapped as private/readonly.
> Mapped private readonly yes, but vm_stat_account() says
> if (file) {
> mm->shared_vm += pages;
> if ((flags & (VM_EXEC|VM_WRITE)) == VM_EXEC)
> mm->exec_vm += pages;

In that code shared_vm includes everything that's mmap()ed, including private
mappings. But if you look at Herbert's patch he has the following change:

if (file) {
- mm->shared_vm += pages;
+ if (flags & VM_SHARED)
+ mm->shared_vm += pages;
if ((flags & (VM_EXEC|VM_WRITE)) == VM_EXEC)
mm->exec_vm += pages;

This means that shared_vm now is truly only memory that's mapped VM_SHARED and
does not include VM_EXEC memory. That necessitates the separate subtraction
of exec_vm in the data calculations.

Dave McCracken
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