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Date:   Mon, 20 May 2019 09:55:33 +0530
From:   Bharata B Rao <bharata@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     srikanth <sraithal@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, bharata@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
        npiggin@...il.com, aneesh.kumar@...ux.ibm.com
Subject: Re: PROBLEM: Power9: kernel oops on memory hotunplug from ppc64le
 guest

On Mon, May 20, 2019 at 12:02:23PM +1000, Michael Ellerman wrote:
> Bharata B Rao <bharata@...ux.ibm.com> writes:
> > On Thu, May 16, 2019 at 07:44:20PM +0530, srikanth wrote:
> >> Hello,
> >> 
> >> On power9 host, performing memory hotunplug from ppc64le guest results in
> >> kernel oops.
> >> 
> >> Kernel used : https://github.com/torvalds/linux/tree/v5.1 built using
> >> ppc64le_defconfig for host and ppc64le_guest_defconfig for guest.
> >> 
> >> Recreation steps:
> >> 
> >> 1. Boot a guest with below mem configuration:
> >>   <maxMemory slots='32' unit='KiB'>33554432</maxMemory>
> >>   <memory unit='KiB'>8388608</memory>
> >>   <currentMemory unit='KiB'>4194304</currentMemory>
> >>   <cpu>
> >>     <numa>
> >>       <cell id='0' cpus='0-31' memory='8388608' unit='KiB'/>
> >>     </numa>
> >>   </cpu>
> >> 
> >> 2. From host hotplug 8G memory -> verify memory hotadded succesfully -> now
> >> reboot guest -> once guest comes back try to unplug 8G memory
> >> 
> >> mem.xml used:
> >> <memory model='dimm'>
> >> <target>
> >> <size unit='GiB'>8</size>
> >> <node>0</node>
> >> </target>
> >> </memory>
> >> 
> >> Memory attach and detach commands used:
> >>     virsh attach-device vm1 ./mem.xml --live
> >>     virsh detach-device vm1 ./mem.xml --live
> >> 
> >> Trace seen inside guest after unplug, guest just hangs there forever:
> >> 
> >> [   21.962986] kernel BUG at arch/powerpc/mm/pgtable-frag.c:113!
> >> [   21.963064] Oops: Exception in kernel mode, sig: 5 [#1]
> >> [   21.963090] LE PAGE_SIZE=64K MMU=Radix MMU=Hash SMP NR_CPUS=2048 NUMA
> >> pSeries
> >> [   21.963131] Modules linked in: xt_tcpudp iptable_filter squashfs fuse
> >> vmx_crypto ib_iser rdma_cm iw_cm ib_cm ib_core libiscsi scsi_transport_iscsi
> >> ip_tables x_tables autofs4 btrfs zstd_decompress zstd_compress lzo_compress
> >> raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx
> >> xor raid6_pq multipath crc32c_vpmsum
> >> [   21.963281] CPU: 11 PID: 316 Comm: kworker/u64:5 Kdump: loaded Not
> >> tainted 5.1.0-dirty #2
> >> [   21.963323] Workqueue: pseries hotplug workque pseries_hp_work_fn
> >> [   21.963355] NIP:  c000000000079e18 LR: c000000000c79308 CTR:
> >> 0000000000008000
> >> [   21.963392] REGS: c0000003f88034f0 TRAP: 0700   Not tainted (5.1.0-dirty)
> >> [   21.963422] MSR:  800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR:
> >> 28002884  XER: 20040000
> >> [   21.963470] CFAR: c000000000c79304 IRQMASK: 0
> >> [   21.963470] GPR00: c000000000c79308 c0000003f8803780 c000000001521000
> >> 0000000000fff8c0
> >> [   21.963470] GPR04: 0000000000000001 00000000ffe30005 0000000000000005
> >> 0000000000000020
> >> [   21.963470] GPR08: 0000000000000000 0000000000000001 c00a000000fff8e0
> >> c0000000016d21a0
> >> [   21.963470] GPR12: c0000000016e7b90 c000000007ff2700 c00a000000a00000
> >> c0000003ffe30100
> >> [   21.963470] GPR16: c0000003ffe30000 c0000000014aa4de c00a0000009f0000
> >> c0000000016d21b0
> >> [   21.963470] GPR20: c0000000014de588 0000000000000001 c0000000016d21b8
> >> c00a000000a00000
> >> [   21.963470] GPR24: 0000000000000000 ffffffffffffffff c00a000000a00000
> >> c0000003ffe96000
> >> [   21.963470] GPR28: c00a000000a00000 c00a000000a00000 c0000003fffec000
> >> c00a000000fff8c0
> >> [   21.963802] NIP [c000000000079e18] pte_fragment_free+0x48/0xd0
> >> [   21.963838] LR [c000000000c79308] remove_pagetable+0x49c/0x5b4
> >> [   21.963873] Call Trace:
> >> [   21.963890] [c0000003f8803780] [c0000003ffe997f0] 0xc0000003ffe997f0
> >> (unreliable)
> >> [   21.963933] [c0000003f88037b0] [0000000000000000] (null)
> >> [   21.963969] [c0000003f88038c0] [c00000000006f038]
> >> vmemmap_free+0x218/0x2e0
> >> [   21.964006] [c0000003f8803940] [c00000000036f100]
> >> sparse_remove_one_section+0xd0/0x138
> >> [   21.964050] [c0000003f8803980] [c000000000383a50]
> >> __remove_pages+0x410/0x560
> >> [   21.964093] [c0000003f8803a90] [c000000000c784d8]
> >> arch_remove_memory+0x68/0xdc
> >> [   21.964136] [c0000003f8803ad0] [c000000000385d74]
> >> __remove_memory+0xc4/0x110
> >> [   21.964180] [c0000003f8803b10] [c0000000000d44e4]
> >> dlpar_remove_lmb+0x94/0x140
> >> [   21.964223] [c0000003f8803b50] [c0000000000d52b4]
> >> dlpar_memory+0x464/0xd00
> >> [   21.964259] [c0000003f8803be0] [c0000000000cd5c0]
> >> handle_dlpar_errorlog+0xc0/0x190
> >> [   21.964303] [c0000003f8803c50] [c0000000000cd6bc]
> >> pseries_hp_work_fn+0x2c/0x60
> >> [   21.964346] [c0000003f8803c80] [c00000000013a4a0]
> >> process_one_work+0x2b0/0x5a0
> >> [   21.964388] [c0000003f8803d10] [c00000000013a818]
> >> worker_thread+0x88/0x610
> >> [   21.964434] [c0000003f8803db0] [c000000000143884] kthread+0x1a4/0x1b0
> >> [   21.964468] [c0000003f8803e20] [c00000000000bdc4]
> >> ret_from_kernel_thread+0x5c/0x78
> >> [   21.964506] Instruction dump:
> >> [   21.964527] fbe1fff8 f821ffd1 78638502 78633664 ebe90000 7fff1a14
> >> 395f0020 813f0020
> >> [   21.964569] 7d2907b4 7d2900d0 79290fe0 69290001 <0b090000> 7c0004ac
> >> 7d205028 3129ffff
> >> [   21.964613] ---[ end trace aaa571aa1636fee6 ]---
> >> [   21.966349]
> >> [   21.966383] Sending IPI to other CPUs
> >> [   21.978335] IPI complete
> >> [   21.981354] kexec: Starting switchover sequence.
> >> I'm in purgatory
> >
> > git bisect points to
> >
> > commit 4231aba000f5a4583dd9f67057aadb68c3eca99d
> > Author: Nicholas Piggin <npiggin@...il.com>
> > 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@...r.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.

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.

Regards,
Bharata.

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