Date:   Wed, 9 Sep 2020 12:59:14 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org, David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>, rafael@...nel.org,
        nathanl@...ux.ibm.com, cheloha@...ux.ibm.com,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations

On Wed 09-09-20 11:21:58, Laurent Dufour wrote:
> Le 09/09/2020 à 11:09, Michal Hocko a écrit :
> > On Wed 09-09-20 09:48:59, Laurent Dufour wrote:
> > > Le 09/09/2020 à 09:40, Michal Hocko a écrit :
[...]
> > > > > In
> > > > > that case, the system is able to boot but later hot-plug operation may lead
> > > > > to this panic because the node's links are correctly broken:
> > > > 
> > > > Correctly broken? Could you provide more details on the inconsistency
> > > > please?
> > > 
> > > laurent@...zep3-lp4:~$ ls -l /sys/devices/system/memory/memory21
> > > total 0
> > > lrwxrwxrwx 1 root root     0 Aug 24 05:27 node1 -> ../../node/node1
> > > lrwxrwxrwx 1 root root     0 Aug 24 05:27 node2 -> ../../node/node2
> > > -rw-r--r-- 1 root root 65536 Aug 24 05:27 online
> > > -r--r--r-- 1 root root 65536 Aug 24 05:27 phys_device
> > > -r--r--r-- 1 root root 65536 Aug 24 05:27 phys_index
> > > drwxr-xr-x 2 root root     0 Aug 24 05:27 power
> > > -r--r--r-- 1 root root 65536 Aug 24 05:27 removable
> > > -rw-r--r-- 1 root root 65536 Aug 24 05:27 state
> > > lrwxrwxrwx 1 root root     0 Aug 24 05:25 subsystem -> ../../../../bus/memory
> > > -rw-r--r-- 1 root root 65536 Aug 24 05:25 uevent
> > > -r--r--r-- 1 root root 65536 Aug 24 05:27 valid_zones
> > 
> > OK, so there are two nodes referenced here. Not terrible from the user
> > point of view. Such a memory block will refuse to offline or online
> > IIRC.
> 
> No the memory block is still owned by one node, only the sysfs
> representation is wrong. So the memory block can be hot unplugged, but only
> one node's link will be cleaned, and a '/syss/devices/system/node#/memory21'
> link will remain and that will be detected later when that memory block is
> hot plugged again.

OK, so you need to hotremove first and hotadd again to trigger the
problem. It is not like you would be a hot adding something new. This is
a useful information to have in the changelog.

> > > > Which physical memory range you are trying to add here and what is the
> > > > node affinity?
> > > 
> > > None is added, the root cause of the issue is happening at boot time.
> > 
> > Let me clarify my question. The crash has clearly happened during the
> > hotplug add_memory_resource - which is clearly not a boot time path.
> > I was askin for more information about why this has failed. It is quite
> > clear that sysfs machinery has failed and that led to BUG_ON but we are
> > mising an information on why. What was the physical memory range to be
> > added and why sysfs failed?
> 
> The BUG_ON is detecting a bad state generated earlier, at boot time because
> register_mem_sect_under_node() didn't check for the block's node id.
> 
> > > > > ------------[ cut here ]------------
> > > > > kernel BUG at /Users/laurent/src/linux-ppc/mm/memory_hotplug.c:1084!
> > > > > Oops: Exception in kernel mode, sig: 5 [#1]
> > > > > LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
> > > > > Modules linked in: rpadlpar_io rpaphp pseries_rng rng_core vmx_crypto gf128mul binfmt_misc ip_tables x_tables xfs libcrc32c crc32c_vpmsum autofs4
> > > > > CPU: 8 PID: 10256 Comm: drmgr Not tainted 5.9.0-rc1+ #25
> > > > > NIP:  c000000000403f34 LR: c000000000403f2c CTR: 0000000000000000
> > > > > REGS: c0000004876e3660 TRAP: 0700   Not tainted  (5.9.0-rc1+)
> > > > > MSR:  800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 24000448  XER: 20040000
> > > > > CFAR: c000000000846d20 IRQMASK: 0
> > > > > GPR00: c000000000403f2c c0000004876e38f0 c0000000012f6f00 ffffffffffffffef
> > > > > GPR04: 0000000000000227 c0000004805ae680 0000000000000000 00000004886f0000
> > > > > GPR08: 0000000000000226 0000000000000003 0000000000000002 fffffffffffffffd
> > > > > GPR12: 0000000088000484 c00000001ec96280 0000000000000000 0000000000000000
> > > > > GPR16: 0000000000000000 0000000000000000 0000000000000004 0000000000000003
> > > > > GPR20: c00000047814ffe0 c0000007ffff7c08 0000000000000010 c0000000013332c8
> > > > > GPR24: 0000000000000000 c0000000011f6cc0 0000000000000000 0000000000000000
> > > > > GPR28: ffffffffffffffef 0000000000000001 0000000150000000 0000000010000000
> > > > > NIP [c000000000403f34] add_memory_resource+0x244/0x340
> > > > > LR [c000000000403f2c] add_memory_resource+0x23c/0x340
> > > > > Call Trace:
> > > > > [c0000004876e38f0] [c000000000403f2c] add_memory_resource+0x23c/0x340 (unreliable)
> > > > > [c0000004876e39c0] [c00000000040408c] __add_memory+0x5c/0xf0
> > > > > [c0000004876e39f0] [c0000000000e2b94] dlpar_add_lmb+0x1b4/0x500
> > > > > [c0000004876e3ad0] [c0000000000e3888] dlpar_memory+0x1f8/0xb80
> > > > > [c0000004876e3b60] [c0000000000dc0d0] handle_dlpar_errorlog+0xc0/0x190
> > > > > [c0000004876e3bd0] [c0000000000dc398] dlpar_store+0x198/0x4a0
> > > > > [c0000004876e3c90] [c00000000072e630] kobj_attr_store+0x30/0x50
> > > > > [c0000004876e3cb0] [c00000000051f954] sysfs_kf_write+0x64/0x90
> > > > > [c0000004876e3cd0] [c00000000051ee40] kernfs_fop_write+0x1b0/0x290
> > > > > [c0000004876e3d20] [c000000000438dd8] vfs_write+0xe8/0x290
> > > > > [c0000004876e3d70] [c0000000004391ac] ksys_write+0xdc/0x130
> > > > > [c0000004876e3dc0] [c000000000034e40] system_call_exception+0x160/0x270
> > > > > [c0000004876e3e20] [c00000000000d740] system_call_common+0xf0/0x27c
> > > > > Instruction dump:
> > > > > 48442e35 60000000 0b030000 3cbe0001 7fa3eb78 7bc48402 38a5fffe 7ca5fa14
> > > > > 78a58402 48442db1 60000000 7c7c1b78 <0b030000> 7f23cb78 4bda371d 60000000
> > > > > ---[ end trace 562fd6c109cd0fb2 ]---
> > > > 
> > > > The BUG_ON on failure is absolutely horrendous. There must be a better
> > > > way to handle a failure like that. The failure means that
> > > > sysfs_create_link_nowarn has failed. Please describe why that is the
> > > > case.
> > > > 
> > > > > This patch addresses the root cause by not relying on the system_state
> > > > > value to detect whether the call is due to a hot-plug operation or not. An
> > > > > additional parameter is added to link_mem_sections() to tell the context of
> > > > > the call and this parameter is propagated to register_mem_sect_under_node()
> > > > > throuugh the walk_memory_blocks()'s call.
> > > > 
> > > > This looks like a hack to me and it deserves a better explanation. The
> > > > existing code is a hack on its own and it is inconsistent with other
> > > > boot time detection. We are using (system_state < SYSTEM_RUNNING) at other
> > > > places IIRC. Would it help to use the same here as well? Maybe we want to
> > > > wrap that inside a helper (early_memory_init()) and use it at all
> > > > places.
> > > 
> > > I agree, this looks like a hack to check for the system_state value.
> > > I'll follow the David's proposal and introduce an enum detailing when the
> > > node id check has to be done or not.
> > 
> > I am not sure an enum is going to make the existing situation less
> > messy. Sure we somehow have to distinguish boot init and runtime hotplug
> > because they have different constrains. I am arguing that a) we should
> > have a consistent way to check for those and b) we shouldn't blow up
> > easily just because sysfs infrastructure has failed to initialize.
> 
> For the point a, using the enum allows to know in
> register_mem_sect_under_node() if the link operation is due to a hotplug
> operation or done at boot time.

Yes, but let me repeat. We have a mess here and different paths check
for the very same condition by different ways. We need to unify those.

> For the point b, one option would be ignore the link error in the case the
> link is already existing, but that BUG_ON() had the benefit to highlight the
> root issue.

Yes BUG_ON is obviously an over-reaction. The system is not in a state
to die anytime soon.
-- 
Michal Hocko
SUSE Labs

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