linux-kernel - Re: [PATCH 02/11] x86/mm/hotplug: Remove pgd

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Date:	Wed, 23 Sep 2015 13:44:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Waiman Long <waiman.long@...com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 02/11] x86/mm/hotplug: Remove pgd_list use from the
	memory hotplug code

On 09/22, Linus Torvalds wrote:
>
> However, this now becomes a pattern for the series, and that just makes me think
>
>     "Why is this not a 'for_each_mm()' pattern helper?"

And we already have other users. And note that oom_kill_process() does _not_
follow this pattern and that is why it is buggy.

So this is funny, but I was thinking about almost the same, something like

	struct task_struct *next_task_with_mm(struct task_struct *p)
	{
		struct task_struct *t;

		p = p->group_leader;
		while ((p = next_task(p)) != &init_task) {
			if (p->flags & PF_KTHREAD)
				continue;

			t = find_lock_task_mm(p);
			if (t)
				return t;
		}

		return NULL;
	}

	#define for_each_task_lock_mm(p)
		for (p = &init_task; (p = next_task_with_mm(p)); task_unlock(p))


So that you can do

	for_each_task_lock_mm(p) {
		do_something_with(p->mm);

		if (some_condition()) {
			// UNFORTUNATELY you can't just do "break"
			task_unlock(p);
			break;
		}
	}

do you think it makes sense?


In fact it can't be simpler, we can move task_unlock() into next_task_with_mm(),
it can check ->mm != NULL or p != init_task.

Oleg.

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