From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bharata B Rao Subject: Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le guest Date: Mon, 20 May 2019 13:50:35 +0530 Message-ID: <20190520082035.GD22939@in.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> <20190520055622.GC22939@in.ibm.com> <1558335484.9inx69a7ea.astroid@bobo.none> Reply-To: bharata@linux.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1558335484.9inx69a7ea.astroid@bobo.none> Sender: linux-kernel-owner@vger.kernel.org To: Nicholas Piggin Cc: 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, Michael Ellerman , srikanth List-Id: linux-next.vger.kernel.org On Mon, May 20, 2019 at 05:00:21PM +1000, Nicholas Piggin wrote: > Bharata B Rao's on May 20, 2019 3:56 pm: > > 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. > > Can't reproduce I'm afraid, tried adding and removing 8GB memory from a > 4GB guest (via host adding / removing memory device), and it just works. Boot, add 8G, reboot, remove 8G is the sequence to reproduce. > > It's likely to be an edge case like an off by one or rounding error > that just happens to trigger in your config. Might be easiest if you > could test with a debug patch. Sure, I will continue debugging. Regards, Bharata.