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Date: Wed, 23 Sep 2015 13:44:53 +0200
From: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 02/11] x86/mm/hotplug: Remove pgd_list use from the
	memory hotplug code
Message-ID: <20150923114453.GA8480@redhat.com>
References: <1442903021-3893-1-git-send-email-mingo@kernel.org> <1442903021-3893-3-git-send-email-mingo@kernel.org> <CA+55aFzN7MMoxzaq-mcNcNoVzUMr0aPHDTipU-OVdaz7_YZ12Q@mail.gmail.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andy Lutomirski <luto@amacapital.net>, Andrew Morton <akpm@linux-foundation.org>, Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, Waiman Long <waiman.long@hp.com>, Thomas Gleixner <tglx@linutronix.de>

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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