Re: [PATCH v1] drivers/base/memory.c: indicate all memory blocks as removable

[David Hildenbrand]

On 27.03.20 17:28, Dan Williams wrote:
> On Fri, Mar 27, 2020 at 2:00 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 27.03.20 08:47, Michal Hocko wrote:
>>> On Thu 26-03-20 23:24:08, Dan Williams wrote:
>>> [...]
>>>> David, Andrew,
>>>>
>>>> I'd like to recommend this patch for -stable as it likely (test
>>>> underway) solves this crash report from Steve:
>>>>
>>>> [  148.796036] page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
>>>> [  148.796074] ------------[ cut here ]------------
>>>> [  148.796098] kernel BUG at include/linux/mm.h:1087!
>>>> [  148.796126] invalid opcode: 0000 [#1] SMP NOPTI
>>>> [  148.796146] CPU: 63 PID: 5471 Comm: lsmem Not tainted 5.5.10-200.fc31.x8=
>>>> 6_64+debug #1
>>>> [  148.796173] Hardware name: Intel Corporation S2600WFD/S2600WFD, BIOS SE5=
>>>> C620.86B.02.01.0010.010620200716 01/06/2020
>>>> [  148.796212] RIP: 0010:is_mem_section_removable+0x1a4/0x1b0
>>>> [  148.796561] Call Trace:
>>>> [  148.796591]  removable_show+0x6e/0xa0
>>>> [  148.796608]  dev_attr_show+0x19/0x40
>>>> [  148.796625]  sysfs_kf_seq_show+0xa9/0x100
>>>> [  148.796640]  seq_read+0xd5/0x450
>>>> [  148.796657]  vfs_read+0xc5/0x180
>>>> [  148.796672]  ksys_read+0x68/0xe0
>>>> [  148.796688]  do_syscall_64+0x5c/0xa0
>>>> [  148.796704]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>> [  148.796721] RIP: 0033:0x7f3ab1646412
>>>>
>>>> ...on a non-debug kernel it just crashes.
>>>>
>>>> In this case lsmem is failing when reading memory96:
>>>>
>>>> openat(3, "memory96/removable", O_RDONLY|O_CLOEXEC) = 4
>>>> fcntl(4, F_GETFL)                       = 0x8000 (flags O_RDONLY|O_LARGEFILE)
>>>> fstat(4, {st_mode=S_IFREG|0444, st_size=4096, ...}) = 0
>>>> read(4,  <unfinished ...>)              = ?
>>>> +++ killed by SIGSEGV +++
>>>> Segmentation fault (core dumped)
>>>>
>>>> ...which is phys_index 0x60 => memory address 0x3000000000
>>>>
>>>> On this platform that lands us here:
>>>>
>>>> 100000000-303fffffff : System RAM
>>>>   291f000000-291fe00f70 : Kernel code
>>>>   2920000000-292051efff : Kernel rodata
>>>>   2920600000-292093b0bf : Kernel data
>>>>   29214f3000-2922dfffff : Kernel bss
>>>> 3040000000-305fffffff : Reserved
>>>> 3060000000-1aa5fffffff : Persistent Memory
>>>
>>> OK, 2GB memblocks and that would mean [0x3000000000, 0x3080000000]
>>>
>>>> ...where the last memory block of System RAM is shared with persistent
>>>> memory. I.e. the block is only partially online which means that
>>>> page_to_nid() in is_mem_section_removable() will assert or crash for
>>>> some of the offline pages in that block.
>>>
>>> Yes, this patch is a simple workaround. Normal memory hotplug will not
>>> blow up because it should be able to find out that test_pages_in_a_zone
>>> is false. Who knows how other potential pfn walkers handle that.
>>
>> All other pfn walkers now correctly use pfn_to_online_page() - which
>> will also result in false positives in this scenario and is still to be
>> fixed by Dan IIRC. [1]
> 
> Sorry, it's been too long and this fell out of my cache. I also turned
> away once the major fire in KVM was put out with special consideration
> for for devmem pages. What's left these days? ...besides
> removable_show()?

Essentially any pfn_to_online_page() is a candidate.

E.g.,

mm/memory-failure.c:memory_failure()

is obviously broken (could be worked around)

Also

mm/memory-failure.c:soft_offline_page()

is obviously broken.


Also set_zone_contiguous()->__pageblock_pfn_to_page() is broken, when it
checks for "page_zone(start_page) != zone" if the memmap contains garbage.

And I only checked a handful of examples.

-- 
Thanks,

David / dhildenb