Date:   Tue, 5 Jun 2018 09:50:33 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Mike Snitzer <snitzer@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        device-mapper development <dm-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: KASAN vs ZONE_DEVICE (was: Re: [PATCH v2 2/7] dax: change bdev_dax_supported()...)

On Tue, Jun 5, 2018 at 6:22 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Mon, Jun 4, 2018 at 8:32 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> [ adding KASAN devs...]
>>
>> On Mon, Jun 4, 2018 at 4:40 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>> On Sun, Jun 3, 2018 at 6:48 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>>> On Sun, Jun 3, 2018 at 5:25 PM, Dave Chinner <david@...morbit.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@...morbit.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?


+kasan-dev

Andrey, can you please take a look at this?

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