From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ve0-f170.google.com (mail-ve0-f170.google.com [209.85.128.170]) by kanga.kvack.org (Postfix) with ESMTP id 0A8E46B0035 for ; Mon, 28 Apr 2014 18:58:03 -0400 (EDT) Received: by mail-ve0-f170.google.com with SMTP id sa20so334707veb.29 for ; Mon, 28 Apr 2014 15:58:03 -0700 (PDT) Received: from mail-vc0-x229.google.com (mail-vc0-x229.google.com [2607:f8b0:400c:c03::229]) by mx.google.com with ESMTPS id ie18si4045255vec.175.2014.04.28.15.58.02 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 28 Apr 2014 15:58:03 -0700 (PDT) Received: by mail-vc0-f169.google.com with SMTP id im17so8955927vcb.14 for ; Mon, 28 Apr 2014 15:58:02 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1398724754.25549.35.camel@buesod1.americas.hpqcorp.net> References: <535EA976.1080402@linux.vnet.ibm.com> <1398724754.25549.35.camel@buesod1.americas.hpqcorp.net> Date: Mon, 28 Apr 2014 15:58:02 -0700 Message-ID: Subject: Re: [BUG] kernel BUG at mm/vmacache.c:85! From: Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Davidlohr Bueso Cc: "Srivatsa S. Bhat" , Linux MM , "linux-kernel@vger.kernel.org" , Rik van Riel , Michel Lespinasse , Hugh Dickins , "akpm@linux-foundation.org" , Oleg Nesterov On Mon, Apr 28, 2014 at 3:39 PM, Davidlohr Bueso wrote: > > Is this perhaps a KVM guest? fwiw I see CONFIG_KVM_ASYNC_PF=y which is a > user of use_mm(). So I tried to look through these guys, and that was one of the ones I looked at. It's using use_mm(), but it's only called through schedule_work(). Which *should* mean that it's in a kernel thread and vmacache_valid_mm() will not be true. HOWEVER. The whole "we don't use the vma cache on kernel threads" does seem to be a pretty fragile approach to the whole workqueue etc issue. I think we always use a kernel thread for workqueue entries, but at the same time I'm not 100% convinced that we should *rely* on that kind of behavior. I don't think that it's necessarily fundamentally guaranteed conceptually - I could see, for example, some user of "flush_work()" deciding to run the work *synchronously* within the context of the process that does the flushing. Now, I don't think we actually do that, but my point is that I think it's a bit dangerous to just say "only kernel threads do use_mm(), and work entries are always done by kernel threads, so let's disable vma caching for kernel threads". It may be *true*, but it's a very indirect kind of true. That's why I think we might be better off saying "let's just invalidate the vmacache in use_mm(), and not care about who does it". No subtle indirect logic about why the caching is safe in one context but not another. But quite frankly, I grepped for things that set "tsk->mm", and apart from clearing it on exit, the only uses I found was copy_mm() (which does that vmacache_flush()) and use_mm(). And all the use_mm() cases _seem_ to be in kernel threads, and that first BUG_ON() didn't have a very complex call chain at all, just a regular page fault from udevd. So it might just be some really nasty corruption totally unrelated to the vmacache, and those preceding odd udevd-work and kdump faults could be related. Linus -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org