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    SubjectRe: [PATCH 2/2] execve: check the VM has enough memory at first
    2010/9/15 KOSAKI Motohiro <>:
    > Briefly says, to introduce new limit has bad benefit/risk balance. Sadly.

    Well, I mostly agree. That said, I do think we could extend the
    limiter some ways.

    For example, I think the "stack limit / 4" is perfectly sane, but it
    would make total sense to perhaps also take into account the AS and
    RSS limits.

    And I do think that your attempt to use __vm_enough_memory() was good.
    It happens to be coded in a way that makes it useless for a one-pass
    model, and some of what it does would be too expensive to do up-front
    when you can't short-circuit it, but I do think that it would probably
    be appropriate to at least try to take the _rough_ code there and use
    it as a limit for maximum stack size too.

    For example, we could have a function somewhat like

    unsigned long max_stack_size(void)
    unsigned long allowed, used, limit;

    switch (sysctl_overcommit_memory) {
    allowed = ULONG_MAX;
    .. maybe we can come up with some upper bound here too ..
    allowed = (totalram_pages - hugetlb_total_pages())
    * sysctl_overcommit_ratio / 100;
    if (!cap_sys_admin)
    allowed -= allowed / 32;
    allowed += total_swap_pages;
    /* Don't let a single process grow too big:
    leave 3% of the size of this process for other processes */
    if (mm)
    allowed -= mm->total_vm / 32;
    /* What is already committed to? */
    used = percpu_counter_read_positive(&vm_committed_as);
    if (used > allowed)
    return 0;
    allowed -= used;
    limit = ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4;
    if (allowed > limit)
    allowed = limit;
    return allowed;

    which we'd call once at the beginning of the execve(), and then
    remember that result and use it instead of the current 'rlimit/4'

    Now, admittedly the OVERCOMMIT_GUESS case is the interesting one, and
    the one that is hard to write efficiently. But maybe we could make
    'nr_free_pages()' cheap enough that doin that whole OVERCOMMIT_GUESS
    "approximate free pages" thing from __vm_enough_memory would work out

    I dunno. It doesn't look hopeless.


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