Bharata B Rao's on May 20, 2019 2:25 pm: > On Mon, May 20, 2019 at 12:02:23PM +1000, Michael Ellerman wrote: >> Bharata B Rao <bharata@xxxxxxxxxxxxx> 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@xxxxxxxxx> >> > 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@xxxxxxxxxxxxxxx # 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. > > 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. Do you have CONFIG_DEBUG_VM=y? Thanks, Nick
