linux-kernel - Re: why do we need vmalloc_sync

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Message-ID: <20150614075943.GA810@gmail.com>
Date:	Sun, 14 Jun 2015 09:59:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, 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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: why do we need vmalloc_sync_all?


* Oleg Nesterov <oleg@...hat.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.

> 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.

Note that all of this will go away if we also do the vmalloc fault handling 
simplification that I discussed with Andy:

 - this series already makes the set of kernel PGDs strictly monotonically 
   increasing during the lifetime of the x86 kernel

 - if in a subsequent patch we can synchronize new PGDs right after the vmalloc
   code creates it, before the area is used - so we can remove vmalloc_fault()
   altogether [or rather, turn it into a debug warning initially].
   vmalloc_fault() is a clever but somewhat fragile complication.

 - after that we can simply remove vmalloc_sync_all() from x86, because all active 
   vmalloc areas will be fully instantiated, all the time, on x86.

Thanks,

	Ingo
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