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    SubjectRe: [PATCH 1/2] mm: set task exit code before complete_vfork_done()
    Oleg Nesterov wrote:
    > On 04/13, Oleg Nesterov wrote:
    >>
    >> Damn, Konstantin I have to admit, I'll try to find another technical
    >> reason against mm-correctly-synchronize-rss-counters-at-exit-exec.patch
    >> even with this fix ;)
    >>
    >> Most probably I am wrong, but it looks overcomplicated. Somehow I
    >> dislike irrationally the fact you moved mm_release() from exit_mm().
    >
    > And perhaps you can help me to discredit your patch?
    >
    > It turns out, I do not really understand this code in do_exit:
    >
    > /* sync mm's RSS info before statistics gathering */
    > if (tsk->mm)
    > sync_mm_rss(tsk->mm);
    >
    > Which "statistics gathering" ? Probably I missed something, but
    > after the quick grep it seems to me that this is only needed for
    > taskstats_exit()->xacct_add_tsk().
    >
    > So why we can't simply add sync_mm_rss() into xacct_add_tsk() ?

    > Yes, this way we do not "account" put_user(clear_child_tid) but
    > I think we do not care.

    Why we don't care? Each thread can corrupt these counters by one.
    I do not think that we are satisfied with nearly accurate rss accounting.
    +/- one page for each clone()-exit().

    Actually I don't really like this per-task rss-delta.
    Probably it would be better to use per-cpu counters.

    >
    > IOW, what do you think about the trivial patch below? Uncompiled,
    > untested, probably incomplete. acct_update_integrals() looks
    > suspicious too.

    what a mess! =)

    >
    > Oleg.
    >
    > --- a/kernel/tsacct.c
    > +++ b/kernel/tsacct.c
    > @@ -91,6 +91,7 @@ void xacct_add_tsk(struct taskstats *sta
    > stats->virtmem = p->acct_vm_mem1 * PAGE_SIZE / MB;
    > mm = get_task_mm(p);
    > if (mm) {
    > + sync_mm_rss(mm);
    > /* adjust to KB unit */
    > stats->hiwater_rss = get_mm_hiwater_rss(mm) * PAGE_SIZE / KB;
    > stats->hiwater_vm = get_mm_hiwater_vm(mm) * PAGE_SIZE / KB;
    > --- a/kernel/exit.c
    > +++ b/kernel/exit.c
    > @@ -643,6 +643,8 @@ static void exit_mm(struct task_struct *
    > mm_release(tsk, mm);
    > if (!mm)
    > return;
    > +
    > + sync_mm_rss(mm);
    > /*
    > * Serialize with any possible pending coredump.
    > * We must hold mmap_sem around checking core_state
    > @@ -960,9 +962,6 @@ void do_exit(long code)
    > preempt_count());
    >
    > acct_update_integrals(tsk);
    > - /* sync mm's RSS info before statistics gathering */
    > - if (tsk->mm)
    > - sync_mm_rss(tsk->mm);
    > group_dead = atomic_dec_and_test(&tsk->signal->live);
    > if (group_dead) {
    > hrtimer_cancel(&tsk->signal->real_timer);
    > --- a/fs/exec.c
    > +++ b/fs/exec.c
    > @@ -823,10 +823,10 @@ static int exec_mmap(struct mm_struct *m
    > /* Notify parent that we're no longer interested in the old VM */
    > tsk = current;
    > old_mm = current->mm;
    > - sync_mm_rss(old_mm);
    > mm_release(tsk, old_mm);
    >
    > if (old_mm) {
    > + sync_mm_rss(old_mm);
    > /*
    > * Make sure that if there is a core dump in progress
    > * for the old mm, we get out and die instead of going
    >
    > --
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