From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-x22b.google.com (mail-wm0-x22b.google.com [IPv6:2a00:1450:400c:c09::22b]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 64F7D1A002B for ; Sat, 13 Feb 2016 02:41:21 +1100 (AEDT) Received: by mail-wm0-x22b.google.com with SMTP id g62so24636038wme.0 for ; Fri, 12 Feb 2016 07:41:21 -0800 (PST) Date: Fri, 12 Feb 2016 17:41:16 +0200 From: "Kirill A. Shutemov" To: Gerald Schaefer Cc: "Kirill A. Shutemov" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Aneesh Kumar K.V" , Andrew Morton , Linus Torvalds , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, Martin Schwidefsky , Heiko Carstens , linux-s390@vger.kernel.org, Sebastian Ott Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) Message-ID: <20160212154116.GA15142@node.shutemov.name> References: <20160211192223.4b517057@thinkpad> <20160211190942.GA10244@node.shutemov.name> <20160211205702.24f0d17a@thinkpad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160211205702.24f0d17a@thinkpad> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Feb 11, 2016 at 08:57:02PM +0100, Gerald Schaefer wrote: > On Thu, 11 Feb 2016 21:09:42 +0200 > "Kirill A. Shutemov" wrote: > > > On Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote: > > > Hi, > > > > > > Sebastian Ott reported random kernel crashes beginning with v4.5-rc1 and > > > he also bisected this to commit 61f5d698 "mm: re-enable THP". Further > > > review of the THP rework patches, which cannot be bisected, revealed > > > commit fecffad "s390, thp: remove infrastructure for handling splitting PMDs" > > > (and also similar commits for other archs). > > > > > > This commit removes the THP splitting bit and also the architecture > > > implementation of pmdp_splitting_flush(), which took care of the IPI for > > > fast_gup serialization. The commit message says > > > > > > pmdp_splitting_flush() is not needed too: on splitting PMD we will do > > > pmdp_clear_flush() + set_pte_at(). pmdp_clear_flush() will do IPI as > > > needed for fast_gup > > > > > > The assumption that a TLB flush will also produce an IPI is wrong on s390, > > > and maybe also on other architectures, and I thought that this was actually > > > the main reason for having an arch-specific pmdp_splitting_flush(). > > > > > > At least PowerPC and ARM also had an individual implementation of > > > pmdp_splitting_flush() that used kick_all_cpus_sync() instead of a TLB > > > flush to send the IPI, and those were also removed. Putting the arch > > > maintainers and mailing lists on cc to verify. > > > > > > On s390 this will break the IPI serialization against fast_gup, which > > > would certainly explain the random kernel crashes, please revert or fix > > > the pmdp_splitting_flush() removal. > > > > Sorry for that. > > > > I believe, the problem was already addressed for PowerPC: > > > > http://lkml.kernel.org/g/454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com > > > > I think kick_all_cpus_sync() in arch-specific pmdp_invalidate() would do > > the trick, right? > > Hmm, not sure about that. After pmdp_invalidate(), a pmd_none() check in > fast_gup will still return false, because the pmd is not empty (at least > on s390). So I don't see spontaneously how it will help fast_gup to break > out to the slow path in case of THP splitting. What pmdp_flush_direct() does in pmdp_invalidate()? It's hard to unwrap for me :-/ Does it make the pmd !pmd_present()? I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm? -- Kirill A. Shutemov