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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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