Messages in this thread | | | From | Dave McCracken <> | Subject | Re: [PATCH] include private data mappings in RLIMIT_DATA limit | Date | Tue, 10 Jul 2007 14:12:19 -0500 |
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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.
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