From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Aneesh Kumar K.V" Subject: Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le guest Date: Mon, 20 May 2019 20:50:17 +0530 Message-ID: <3ec09ffa-e3d8-5ba2-cf1e-c0d47cfc2332@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> <20190520055622.GC22939@in.ibm.com> <1558335484.9inx69a7ea.astroid@bobo.none> <20190520082035.GD22939@in.ibm.com> <20190520142922.GE22939@in.ibm.com> <1558363500.jsgl4a2lfa.astroid@bobo.none> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1558363500.jsgl4a2lfa.astroid@bobo.none> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Nicholas Piggin , bharata@linux.ibm.com Cc: 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 5/20/19 8:25 PM, Nicholas Piggin wrote: > Bharata B Rao's on May 21, 2019 12:29 am: >> On Mon, May 20, 2019 at 01:50:35PM +0530, Bharata B Rao wrote: >>> 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. >> >> When the guest is rebooted after hotplug, the entire memory (which includes >> the hotplugged memory) gets remapped again freshly. However at this time >> since no slab is available yet, pt_frag_refcount never gets initialized as we >> never do pte_fragment_alloc() for these mappings. So we right away hit the >> underflow during the first unplug itself, it looks like. > > Nice catch, good debugging work. > >> I will check how this can be fixed. > > Tricky problem. What do you think? You might be able to make the early > page table allocations in the same pattern as the frag allocations, and > then fill in the struct page metadata when you have those. I guess we need to do something similar to what x86 does. We need to walk the init_mm page table again and re-init struct page and other data structures backing the tables? -aneesh