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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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