From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id AAFF06B0315 for ; Thu, 8 Jun 2017 16:30:51 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id w91so5981070wrb.13 for ; Thu, 08 Jun 2017 13:30:51 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id k24si6926689wmh.19.2017.06.08.13.30.49 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 08 Jun 2017 13:30:50 -0700 (PDT) Date: Thu, 8 Jun 2017 22:30:47 +0200 From: Michal Hocko Subject: Re: Sleeping BUG in khugepaged for i586 Message-ID: <20170608203046.GB5535@dhcp22.suse.cz> References: <968ae9a9-5345-18ca-c7ce-d9beaf9f43b6@lwfinger.net> <20170605144401.5a7e62887b476f0732560fa0@linux-foundation.org> <1e883924-9766-4d2a-936c-7a49b337f9e2@lwfinger.net> <9ab81c3c-e064-66d2-6e82-fc9bac125f56@suse.cz> <20170608144831.GA19903@dhcp22.suse.cz> <20170608170557.GA8118@bombadil.infradead.org> <20170608201822.GA5535@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170608201822.GA5535@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: David Rientjes , Vlastimil Babka , Larry Finger , Andrew Morton , LKML , linux-mm@kvack.org On Thu 08-06-17 22:18:22, Michal Hocko wrote: > On Thu 08-06-17 10:05:57, Matthew Wilcox wrote: > > On Thu, Jun 08, 2017 at 04:48:31PM +0200, Michal Hocko wrote: > > > On Wed 07-06-17 13:56:01, David Rientjes wrote: > > > > I agree it's probably going to bisect to 338a16ba15495 since it's the > > > > cond_resched() at the line number reported, but I think there must be > > > > something else going on. I think the list of locks held by khugepaged is > > > > correct because it matches with the implementation. The preempt_count(), > > > > as suggested by Andrew, does not. If this is reproducible, I'd like to > > > > know what preempt_count() is. > > > > > > collapse_huge_page > > > pte_offset_map > > > kmap_atomic > > > kmap_atomic_prot > > > preempt_disable > > > __collapse_huge_page_copy > > > pte_unmap > > > kunmap_atomic > > > __kunmap_atomic > > > preempt_enable > > > > > > I suspect, so cond_resched seems indeed inappropriate on 32b systems. > > > > Then why doesn't it trigger on 64-bit systems too? > > > > #ifndef ARCH_HAS_KMAP > > ... > > static inline void *kmap_atomic(struct page *page) > > { > > preempt_disable(); > > pagefault_disable(); > > return page_address(page); > > } > > #define kmap_atomic_prot(page, prot) kmap_atomic(page) > > > > > > ... oh, wait, I see. Because pte_offset_map() doesn't call kmap_atomic() > > on 64-bit. Indeed, it doesn't necessarily call kmap_atomic() on 32-bit > > either; only with CONFIG_HIGHPTE enabled. How much of a performance > > penalty would it be to call kmap_atomic() unconditionally on 64 bit to > > make sure that this kind of problem doesn't show on 32-bit systems only? > > I am not sure I understand why would we map those pages in 64b systems? > We can access them directly. But I guess you are primary after syncing the preemptive mode for 64 and 32b systems, right? I agree that having a different model is more than unfortunate because 32b gets much less testing coverage and so a risk of introducing a new bug is just a matter of time. Maybe we should make pte_offset_map disable preemption and currently noop pte_unmap to preempt_enable. The overhead should be pretty marginal on x86_64 but not all arches have per-cpu preempt count. So I am not sure we really want to add this to just for the debugging purposes... I would just pull the cond_resched out of __collapse_huge_page_copy right after pte_unmap. But I am not really sure why this cond_resched is really needed because the changelog of the patch which adds is is quite terse on details. -- Michal Hocko SUSE Labs -- 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