linux-kernel - Re: [PATCH -next] mm,vmacache: also flush cache for VM

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Date:	Thu, 13 Mar 2014 12:04:17 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Davidlohr Bueso <davi@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] mm,vmacache: also flush cache for VM_CLONE

On Thu, 2014-03-13 at 16:32 +0100, Oleg Nesterov wrote:
> On 03/13, Oleg Nesterov wrote:
> >
> > Yes. But it seems that use_mm() and unuse_mm() should invalidate vmacache too.
> >
> > Suppose that a kernel thread T does, say,
> >
> > 	use_mm(foreign_mm);
> > 	get_user(...);
> > 	unuse_mm();
> >
> > This can trigger a fault and populate T->vmacache[]. If this code is called
> > again vmacache_find() can use the stale entries.
> >
> > Or, assuming that only a kernel thread can do use_mm(), we can change
> > vmacache_valid() to also check !PF_KTHREAD.
> 
> Yes, I think we should check PF_KTHREAD, because
> 
> > Hmm. Another problem is that use_mm() doesn't take ->mmap_sem and thus
> > it can race with vmacache_flush_all()...
> 
> this also closes this race. use_mm() users should not use vmacache at all.
> 
> > Finally. Shouldn't vmacache_update() check current->mm == mm as well?
> > What if access_remote_vm/get_user_pages trigger find_vma() ??? Unless
> > I missed something this is not theoretical at all and can lead to the
> > corrupted vmacache, no?
> 
> Looks like a real problem or I am totally confused. I think we need
> something like below (uncompiled).

Thanks for looking into this Oleg. I was actually chasing a bug
triggered by trinity where we have a stale cache and vmacache_find() is
returning a bogus vma structure even when vma->vm_mm != mm:

https://lkml.org/lkml/2014/3/9/201
https://lkml.org/lkml/2014/3/11/563

So it just might be a real problem.

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