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From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, 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>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Waiman Long <Waiman.Long@hp.com>
Subject: Re: why do we need vmalloc_sync_all?
Date: Sun, 14 Jun 2015 22:06:23 +0200	[thread overview]
Message-ID: <20150614200623.GB19582@redhat.com> (raw)
In-Reply-To: <20150614075943.GA810@gmail.com>

On 06/14, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > I didn't read v2 yet, but I'd like to ask a question.
> >
> > Why do we need vmalloc_sync_all()?
> >
> > It has a single caller, register_die_notifier() which calls it without
> > any explanation. IMO, this needs a comment at least.
>
> Yes, it's used to work around crashes in modular callbacks: if the callbacks
> happens to be called from within the page fault path, before the vmalloc page
> fault handler runs, then we have a catch-22 problem.
>
> It's rare but not entirely impossible.

But again, the kernel no longer does this? do_page_fault() does vmalloc_fault()
without notify_die(). If it fails, I do not see how/why a modular DIE_OOPS
handler could try to resolve this problem and trigger another fault.

> > I am not sure I understand the changelog in 101f12af correctly, but at first
> > glance vmalloc_sync_all() is no longer needed at least on x86, do_page_fault()
> > no longer does notify_die(DIE_PAGE_FAULT). And btw DIE_PAGE_FAULT has no users.
> > DIE_MNI too...
> >
> > Perhaps we can simply kill it on x86?
>
> So in theory we could still have it run from DIE_OOPS, and that could turn a
> survivable kernel crash into a non-survivable one.

I don't understand... But OK, my understanding of this magic is very limited,
please forget.

Thanks,

Oleg.

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  reply	other threads:[~2015-06-14 20:07 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-06-13  9:49 [PATCH 00/12, v2] x86/mm: Implement lockless pgd_alloc()/pgd_free() Ingo Molnar
2015-06-13  9:49 ` [PATCH 01/12] x86/mm/pat: Don't free PGD entries on memory unmap Ingo Molnar
2015-06-13  9:49 ` [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code Ingo Molnar
2015-06-13 19:24   ` Oleg Nesterov
2015-06-14  7:36     ` Ingo Molnar
2015-06-14 19:24       ` Oleg Nesterov
2015-06-14 19:38         ` Oleg Nesterov
2015-06-15  0:40           ` Paul E. McKenney
2015-06-15 20:33             ` Ingo Molnar
2015-06-13  9:49 ` [PATCH 03/12] x86/mm/hotplug: Don't remove PGD entries in remove_pagetable() Ingo Molnar
2015-06-13  9:49 ` [PATCH 04/12] x86/mm/hotplug: Simplify sync_global_pgds() Ingo Molnar
2015-06-13  9:49 ` [PATCH 05/12] mm: Introduce arch_pgd_init_late() Ingo Molnar
2015-06-13  9:49 ` [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late() method Ingo Molnar
2015-06-13  9:49 ` [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code Ingo Molnar
2015-06-14  8:26   ` Ingo Molnar
2015-06-15  9:05   ` Ian Campbell
2015-06-15 10:30     ` David Vrabel
2015-06-15 20:35       ` Ingo Molnar
2015-06-16 14:15         ` David Vrabel
2015-06-16 14:19           ` Boris Ostrovsky
2015-06-16 14:27             ` David Vrabel
2015-06-13  9:49 ` [PATCH 08/12] x86/mm: Remove pgd_list use from vmalloc_sync_all() Ingo Molnar
2015-06-13  9:49 ` [PATCH 09/12] x86/mm/pat/32: Remove pgd_list use from the PAT code Ingo Molnar
2015-06-13  9:49 ` [PATCH 10/12] x86/mm: Make pgd_alloc()/pgd_free() lockless Ingo Molnar
2015-06-13  9:49 ` [PATCH 11/12] x86/mm: Remove pgd_list leftovers Ingo Molnar
2015-06-13  9:49 ` [PATCH 12/12] x86/mm: Simplify pgd_alloc() Ingo Molnar
2015-06-13 18:58 ` why do we need vmalloc_sync_all? Oleg Nesterov
2015-06-14  7:59   ` Ingo Molnar
2015-06-14 20:06     ` Oleg Nesterov [this message]
2015-06-15  2:47       ` Andi Kleen
2015-06-15  2:57         ` Andy Lutomirski
2015-06-15 20:28           ` Ingo Molnar
2015-06-15 20:48             ` Andy Lutomirski

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