linux-kernel - Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd

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Message-ID: <20150612203832.GA18966@redhat.com>
Date:	Fri, 12 Jun 2015 22:38:32 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-mml@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
	the Xen guest code

On 06/12, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
> > So I think the only issue is that ->mm can become NULL when the thread group
> > leader dies - a non-NULL mm should always be shared among all threads.
>
> Indeed, we do that in exit_mm().

Yes,

> So we could add tsk->mm_leader or so,

No, no, please do not. Just do something like

	for_each_process(p) {

		for_each_thread(p, t) {
			if (t->mm) {
				do_something(t->mm);
				break;
			}
		}
	}

But either way I don't understand what protects this ->mm. Perhaps this needs
find_lock_task_mm().

Oleg.

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