[dm-devel] KASAN vs ZONE_DEVICE (was: Re: [PATCH v2 2/7] dax: change bdev_dax_supported()...)

Tue Jun 5 04:22:15 UTC 2018

On Mon, Jun 4, 2018 at 8:32 PM, Dan Williams <dan.j.williams at intel.com> wrote:
> [ adding KASAN devs...]
>
> On Mon, Jun 4, 2018 at 4:40 PM, Dan Williams <dan.j.williams at intel.com> wrote:
>> On Sun, Jun 3, 2018 at 6:48 PM, Dan Williams <dan.j.williams at intel.com> wrote:
>>> On Sun, Jun 3, 2018 at 5:25 PM, Dave Chinner <david at fromorbit.com> wrote:
>>>> On Mon, Jun 04, 2018 at 08:20:38AM +1000, Dave Chinner wrote:
>>>>> On Thu, May 31, 2018 at 09:02:52PM -0700, Dan Williams wrote:
>>>>> > On Thu, May 31, 2018 at 7:24 PM, Dave Chinner <david at fromorbit.com> wrote:
>>>>> > > On Thu, May 31, 2018 at 06:57:33PM -0700, Dan Williams wrote:
>>>>> > >> > FWIW, XFS+DAX used to just work on this setup (I hadn't even
>>>>> > >> > installed ndctl until this morning!) but after changing the kernel
>>>>> > >> > it no longer works. That would make it a regression, yes?
>>>>>
>>>>> [....]
>>>>>
>>>>> > >> I suspect your kernel does not have CONFIG_ZONE_DEVICE enabled which
>>>>> > >> has the following dependencies:
>>>>> > >>
>>>>> > >>         depends on MEMORY_HOTPLUG
>>>>> > >>         depends on MEMORY_HOTREMOVE
>>>>> > >>         depends on SPARSEMEM_VMEMMAP
>>>>> > >
>>>>> > > Filesystem DAX now has a dependency on memory hotplug?
>>>>>
>>>>> [....]
>>>>>
>>>>> > > OK, works now I've found the magic config incantantions to turn
>>>>> > > everything I now need on.
>>>>>
>>>>> By enabling these options, my test VM now has a ~30s pause in the
>>>>> boot very soon after the nvdimm subsystem is initialised.
>>>>>
>>>>> [    1.523718] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
>>>>> [    1.550353] 00:05: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
>>>>> [    1.552175] Non-volatile memory driver v1.3
>>>>> [    2.332045] tsc: Refined TSC clocksource calibration: 2199.909 MHz
>>>>> [    2.333280] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x1fb5dcd4620, max_idle_ns: 440795264143 ns
>>>>> [   37.217453] brd: module loaded
>>>>> [   37.225423] loop: module loaded
>>>>> [   37.228441] virtio_blk virtio2: [vda] 10485760 512-byte logical blocks (5.37 GB/5.00 GiB)
>>>>> [   37.245418] virtio_blk virtio3: [vdb] 146800640 512-byte logical blocks (75.2 GB/70.0 GiB)
>>>>> [   37.255794] virtio_blk virtio4: [vdc] 1073741824000 512-byte logical blocks (550 TB/500 TiB)
>>>>> [   37.265403] nd_pmem namespace1.0: unable to guarantee persistence of writes
>>>>> [   37.265618] nd_pmem namespace0.0: unable to guarantee persistence of writes
>>>>>
>>>>> The system does not appear to be consuming CPU, but it is blocking
>>>>> NMIs so I can't get a CPU trace. For a VM that I rely on booting in
>>>>> a few seconds because I reboot it tens of times a day, this is a
>>>>> problem....
>>>>
>>>> And when I turn on KASAN, the kernel fails to boot to a login prompt
>>>> because:
>>>
>>> What's your qemu and kernel command line? I'll take look at this first
>>> thing tomorrow.
>>
>> I was able to reproduce this crash by just turning on KASAN...
>> investigating. It would still help to have your config for our own
>> regression testing purposes it makes sense for us to prioritize
>> "Dave's test config", similar to the priority of not breaking Linus'
>> laptop.
>
> I believe this is a bug in KASAN, or a bug in devm_memremap_pages(),
> depends on your point of view. At the very least it is a mismatch of
> assumptions. KASAN learns of hot added memory via the memory hotplug
> notifier. However, the devm_memremap_pages() implementation is
> intentionally limited to the "first half" of the memory hotplug
> procedure. I.e. it does just enough to setup the linear map for
> pfn_to_page() and initialize the "struct page" memmap, but then stops
> short of onlining the pages. This is why we are getting a NULL ptr
> deref and not a KASAN report, because KASAN has no shadow area setup
> for the linearly mapped pmem range.
>
> In terms of solving it we could refactor kasan_mem_notifier() so that
> devm_memremap_pages() can call it outside of the notifier... I'll give
> this a shot.

Well, the attached patch got me slightly further, but only slightly...

[   14.998394] BUG: KASAN: unknown-crash in pmem_do_bvec+0x19e/0x790 [nd_pmem]
[   15.000006] Read of size 4096 at addr ffff880200000000 by task
systemd-udevd/915
[   15.001991]
[   15.002590] CPU: 15 PID: 915 Comm: systemd-udevd Tainted: G
  OE     4.17.0-rc5+ #1
982
[   15.004783] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.11.1-0-g0551a
4be2c-prebuilt.qemu-project.org 04/01/2014
[   15.007652] Call Trace:
[   15.008339]  dump_stack+0x9a/0xeb
[   15.009344]  print_address_description+0x73/0x280
[   15.010524]  kasan_report+0x258/0x380
[   15.011528]  ? pmem_do_bvec+0x19e/0x790 [nd_pmem]
[   15.012747]  memcpy+0x1f/0x50
[   15.013659]  pmem_do_bvec+0x19e/0x790 [nd_pmem]

...I've exhausted my limited kasan internals knowledge, any ideas what
it's missing?
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