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

[Michal Hocko]

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.

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.

That being said, I do not have any objections to backporting to stable
trees.
-- 
Michal Hocko
SUSE Labs