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

David Hildenbrand
2020-03-27T09:00:26Z

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]

>
> Risking to sound like a broken record I will remind that I have been
> pushing for having _all_ existing struct pages initialized and we
> wouldn't have problems like this popping out here and there.

The real issue is that we have uninitialized memmap within a section
that is marked to contain initialized/online memmap. As I said, even
pfn_to_online_page()/SECTION_IS_ONLINE does not help here.

Also, there is no way to mark devmem to have a initialized memmap
(something like SECTION_IS_ONLINE). I expressed my feeling about that
already.

[1] contains a discussion how it could be addressed.

>
> That being said, I do not have any objections to backporting to stable
> trees.
>

This one is one of the remaining places where we don't use
pfn_to_online_page(). So yeah, this patch shouldn't hurt.

[1] https://lkml.kernel.org/r/20191024120938.11237-1-david@redhat.com

--
Thanks,

David / dhildenb