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

[Dan Williams]

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()?