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 20:42:02 +0530 Message-ID: <20190520151202.GF22939@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> <20190520082035.GD22939@in.ibm.com> <20190520142922.GE22939@in.ibm.com> <1558363500.jsgl4a2lfa.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: <1558363500.jsgl4a2lfa.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 Tue, May 21, 2019 at 12:55:49AM +1000, 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. Thanks, with help from Aneesh. > > > 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. Will explore. > > Other option may be create a new set of page tables after mm comes up > to replace the early page tables with. That's a bigger hammer though. Will also check if similar scenario exists on x86 and if so, how and when pte frag data is fixed there. Regards, Bharata.