linux-kernel - Re: [PATCH 1/2] mm: set task exit code before complete_vfork

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Date:	Fri, 20 Apr 2012 19:59:34 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Markus Trippelsdorf <markus@...ppelsdorf.de>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 1/2] mm: set task exit code before complete_vfork_done()

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.

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

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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