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[193.116.79.244]) by smtp.gmail.com with ESMTPSA id t7sm20650667pfh.156.2019.05.20.07.56.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 20 May 2019 07:56:53 -0700 (PDT) Date: Tue, 21 May 2019 00:55:49 +1000 From: Nicholas Piggin Subject: Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le guest To: bharata@linux.ibm.com 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 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> In-Reply-To: <20190520142922.GE22939@in.ibm.com> MIME-Version: 1.0 User-Agent: astroid/0.14.0 (https://github.com/astroidmail/astroid) Message-Id: <1558363500.jsgl4a2lfa.astroid@bobo.none> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 spec= ulative references >> > >> >> > >> > >> >> > The page table fragment allocator uses the main page refco= unt racily >> > >> >> > with respect to speculative references. A customer observe= d a BUG due >> > >> >> > to page table page refcount underflow in the fragment allo= cator. This >> > >> >> > can be caused by the fragment allocator set_page_count sto= mping on a >> > >> >> > speculative reference, and then the speculative failure ha= ndler >> > >> >> > 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 was= tage") >> > >> >> > Cc: stable@vger.kernel.org # v3.10+ >> > >> >>=20 >> > >> >> That's the commit that added the BUG_ON(), so prior to that you = won't >> > >> >> see the crash. >> > >> >=20 >> > >> > Right, but the commit says it fixes page table page refcount unde= rflow by >> > >> > introducing a new field &page->pt_frag_refcount. Now we are hitti= ng the underflow >> > >> > for this pt_frag_refcount. >> > >>=20 >> > >> The fixed underflow is caused by a bug (race on page count) that go= t=20 >> > >> 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. >> > >=20 >> > > Ok. >> >=20 >> > 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 work= s. >>=20 >> Boot, add 8G, reboot, remove 8G is the sequence to reproduce. >>=20 >> >=20 >> > 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. >>=20 >> Sure, I will continue debugging. >=20 > When the guest is rebooted after hotplug, the entire memory (which includ= es > the hotplugged memory) gets remapped again freshly. However at this time > since no slab is available yet, pt_frag_refcount never gets initialized a= s we > never do pte_fragment_alloc() for these mappings. So we right away hit th= e > 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=20 page table allocations in the same pattern as the frag allocations, and=20 then fill in the struct page metadata when you have those. 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. Thanks, Nick =