From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com (foss.arm.com [217.140.101.70]) by lists.ozlabs.org (Postfix) with ESMTP id B29701A002B for ; Fri, 12 Feb 2016 21:01:37 +1100 (AEDT) Date: Fri, 12 Feb 2016 10:01:37 +0000 From: Will Deacon To: Gerald Schaefer Cc: "Kirill A. Shutemov" , "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 , 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: <20160212100137.GE25087@arm.com> 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: > > > 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. > > > > > If yes, I'll prepare patch tomorrow (some sleep required). > > > > We'll check if adding kick_all_cpus_sync() to pmdp_invalidate() helps. > It would also be good if Martin has a look at this, he'll return on > Monday. Do you have a reliable way to trigger the "random kernel crashes"? We've not seen anything reported on arm64, but I don't see why we wouldn't be affected by the same bug and it would be good to confirm and validate a fix. Cheers, Will