Re: [PATCH 1/2] mm: set task exit code before complete_vfork_done()

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

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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