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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 20 May 2019 06:56:30 +0100 Received: from d06av23.portsmouth.uk.ibm.com (d06av23.portsmouth.uk.ibm.com [9.149.105.59]) by b06cxnps4076.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x4K5uTST24838354 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 20 May 2019 05:56:29 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C65D4A404D; Mon, 20 May 2019 05:56:29 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5FDF4A4057; Mon, 20 May 2019 05:56:28 +0000 (GMT) Received: from in.ibm.com (unknown [9.124.35.55]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTPS; Mon, 20 May 2019 05:56:28 +0000 (GMT) Date: Mon, 20 May 2019 11:26:22 +0530 From: Bharata B Rao To: Nicholas Piggin Cc: Michael Ellerman , aneesh.kumar@linux.ibm.com, bharata@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, linux-next@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, srikanth Subject: Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le guest Reply-To: bharata@linux.ibm.com References: <16a7a635-c592-27e2-75b4-d02071833278@linux.vnet.ibm.com> <20190518141434.GA22939@in.ibm.com> <878sv1993k.fsf@concordia.ellerman.id.au> <20190520042533.GB22939@in.ibm.com> <1558327521.633yjtl8ki.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1558327521.633yjtl8ki.astroid@bobo.none> User-Agent: Mutt/1.10.1 (2018-07-13) X-TM-AS-GCONF: 00 x-cbid: 19052005-0020-0000-0000-0000033E71BD X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19052005-0021-0000-0000-0000219145CD Message-Id: <20190520055622.GC22939@in.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-05-20_03:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905200043 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 20, 2019 at 02:48:35PM +1000, Nicholas Piggin wrote: > >> > git bisect points to > >> > > >> > commit 4231aba000f5a4583dd9f67057aadb68c3eca99d > >> > Author: Nicholas Piggin > >> > Date: Fri Jul 27 21:48:17 2018 +1000 > >> > > >> > powerpc/64s: Fix page table fragment refcount race vs speculative references > >> > > >> > The page table fragment allocator uses the main page refcount racily > >> > with respect to speculative references. A customer observed a BUG due > >> > to page table page refcount underflow in the fragment allocator. This > >> > can be caused by the fragment allocator set_page_count stomping on a > >> > speculative reference, and then the speculative failure handler > >> > decrements the new reference, and the underflow eventually pops when > >> > the page tables are freed. > >> > > >> > Fix this by using a dedicated field in the struct page for the page > >> > table fragment allocator. > >> > > >> > Fixes: 5c1f6ee9a31c ("powerpc: Reduce PTE table memory wastage") > >> > Cc: stable@vger.kernel.org # v3.10+ > >> > >> That's the commit that added the BUG_ON(), so prior to that you won't > >> see the crash. > > > > Right, but the commit says it fixes page table page refcount underflow by > > introducing a new field &page->pt_frag_refcount. Now we are hitting the underflow > > for this pt_frag_refcount. > > The fixed underflow is caused by a bug (race on page count) that got > fixed by that patch. You are hitting a different underflow here. It's > not certain my patch caused it, I'm just trying to reproduce now. Ok. > > > > > BTW, if I go below this commit, I don't hit the pagecount > > > > VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); > > > > which is in pte_fragment_free() path. > > Do you have CONFIG_DEBUG_VM=y? Yes. Regards, Bharata.