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

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Message-ID: <20150929084238.GA332@gmail.com>
Date:	Tue, 29 Sep 2015 10:42:38 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.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


* Oleg Nesterov <oleg@...hat.com> wrote:

> 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?

Sure, I'm inclined to use the above code from you.

> 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.

s/can't/can ?

But even with that I'm not sure I can parse your suggestion. Got some (pseudo) code
perhaps?

Thanks,

	Ingo
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