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Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations
To:     Michal Hocko <mhocko@suse.com>
Cc:     akpm@linux-foundation.org, David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>, rafael@kernel.org,
        nathanl@linux.ibm.com, cheloha@linux.ibm.com,
        stable@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
References: <5cbd92e1-c00a-4253-0119-c872bfa0f2bc@redhat.com>
 <20200908170835.85440-1-ldufour@linux.ibm.com>
 <20200909074011.GD7348@dhcp22.suse.cz>
 <9faac1ce-c02d-7dbc-f79a-4aaaa5a73d28@linux.ibm.com>
 <20200909090953.GE7348@dhcp22.suse.cz>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Message-ID: <4cdb54be-1a92-4ba4-6fee-3b415f3468a9@linux.ibm.com>
Date:   Wed, 9 Sep 2020 11:21:58 +0200
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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 :
>>> [reposting because the malformed cc list confused my email client]
>>>
>>> On Tue 08-09-20 19:08:35, Laurent Dufour wrote:
>>>> In register_mem_sect_under_node() the system_state’s value is checked to
>>>> detect whether the operation the call is made during boot time or during an
>>>> hot-plug operation. Unfortunately, that check is wrong on some
>>>> architecture, and may lead to sections being registered under multiple
>>>> nodes if node's memory ranges are interleaved.
>>>
>>> Why is this check arch specific?
>>
>> I was wrong the check is not arch specific.
>>
>>>> This can be seen on PowerPC LPAR after multiple memory hot-plug and
>>>> hot-unplug operations are done. At the next reboot the node's memory ranges
>>>> can be interleaved
>>>
>>> What is the exact memory layout?
>>
>> For instance:
>> [    0.000000] Early memory node ranges
>> [    0.000000]   node   1: [mem 0x0000000000000000-0x000000011fffffff]
>> [    0.000000]   node   2: [mem 0x0000000120000000-0x000000014fffffff]
>> [    0.000000]   node   1: [mem 0x0000000150000000-0x00000001ffffffff]
>> [    0.000000]   node   0: [mem 0x0000000200000000-0x000000048fffffff]
>> [    0.000000]   node   2: [mem 0x0000000490000000-0x00000007ffffffff]
> 
> Include this into the changelog.
> 
>>>> and since the call to link_mem_sections() is made in
>>>> topology_init() while the system is in the SYSTEM_SCHEDULING state, the
>>>> node's id is not checked, and the sections registered multiple times.
>>>
>>> So a single memory section/memblock belongs to two numa nodes?
>>
>> If the node id is not checked in register_mem_sect_under_node(), yes that the case.
> 
> I do not follow. register_mem_sect_under_node is about user interface.
> This is independent on the low level memory representation - aka memory
> section. I do not think we can handle a section in multiple zones/nodes.
> Memblock in multiple zones/nodes is a different story and interleaving
> physical memory layout can indeed lead to it. This is something that we
> do not allow for runtime hotplug but have to somehow live with that - at
> least not crash.

register_mem_sect_under_node() is called at boot time and when memory is hot 
added. In the later case the assumption is made that all the pages of the added 
block are in the same node. And that's a valid assumption. However at boot time 
the call is made using the node's whole range, lowest address to highest address 
for that node. In the case there are interleaved ranges, this means the 
interleaved sections are registered for each nodes which is not correct.

>>>> 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@ltczep3-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.

>   
>>> 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.

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.

Cheers,
Laurent.