Re: dax pmd fault handler never returns to userspace

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Wed, Nov 18, 2015 at 10:10:45AM -0800, Dan Williams wrote:
> On Wed, Nov 18, 2015 at 9:43 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> > Ross Zwisler <ross.zwisler@linux.intel.com> writes:
> >
> >> On Wed, Nov 18, 2015 at 08:52:59AM -0800, Dan Williams wrote:
> >>> Sysrq-t or sysrq-w dump?  Also do you have the locking fix from Yigal?
> >>>
> >>> https://lists.01.org/pipermail/linux-nvdimm/2015-November/002842.html
> >>
> >> I was able to reproduce the issue in my setup with v4.3, and the patch from
> >> Yigal seems to solve it.  Jeff, can you confirm?
> >
> > I applied the patch from Yigal and the symptoms persist.  Ross, what are
> > you testing on?  I'm using an NVDIMM-N.
> >
> > Dan, here's sysrq-l (which is what w used to look like, I think).  Only
> > cpu 3 is interesting:
> >
> > [  825.339264] NMI backtrace for cpu 3
> > [  825.356347] CPU: 3 PID: 13555 Comm: blk_non_zero.st Not tainted 4.4.0-rc1+ #17
> > [  825.392056] Hardware name: HP ProLiant DL380 Gen9, BIOS P89 06/09/2015
> > [  825.424472] task: ffff880465bf6a40 ti: ffff88046133c000 task.ti: ffff88046133c000
> > [  825.461480] RIP: 0010:[<ffffffff81329856>]  [<ffffffff81329856>] strcmp+0x6/0x30
> > [  825.497916] RSP: 0000:ffff88046133fbc8  EFLAGS: 00000246
> > [  825.524836] RAX: 0000000000000000 RBX: ffff880c7fffd7c0 RCX: 000000076c800000
> > [  825.566847] RDX: 000000076c800fff RSI: ffffffff818ea1c8 RDI: ffffffff818ea1c8
> > [  825.605265] RBP: ffff88046133fbc8 R08: 0000000000000001 R09: ffff8804652300c0
> > [  825.643628] R10: 00007f1b4fe0b000 R11: ffff880465230228 R12: ffffffff818ea1bd
> > [  825.681381] R13: 0000000000000001 R14: ffff88046133fc20 R15: 0000000080000200
> > [  825.718607] FS:  00007f1b5102d880(0000) GS:ffff88046f8c0000(0000) knlGS:00000000000000
> > 00
> > [  825.761663] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [  825.792213] CR2: 00007f1b4fe0b000 CR3: 000000046b225000 CR4: 00000000001406e0
> > [  825.830906] Stack:
> > [  825.841235]  ffff88046133fc10 ffffffff81084610 000000076c800000 000000076c800fff
> > [  825.879533]  000000076c800fff 00000000ffffffff ffff88046133fc90 ffffffff8106d1d0
> > [  825.916774]  000000000000000c ffff88046133fc80 ffffffff81084f0d 000000076c800000
> > [  825.953220] Call Trace:
> > [  825.965386]  [<ffffffff81084610>] find_next_iomem_res+0xd0/0x130
> > [  825.996804]  [<ffffffff8106d1d0>] ? pat_enabled+0x20/0x20
> > [  826.024773]  [<ffffffff81084f0d>] walk_system_ram_range+0x8d/0xf0
> > [  826.055565]  [<ffffffff8106d2d8>] pat_pagerange_is_ram+0x78/0xa0
> > [  826.088971]  [<ffffffff8106d475>] lookup_memtype+0x35/0xc0
> > [  826.121385]  [<ffffffff8106e33b>] track_pfn_insert+0x2b/0x60
> > [  826.154600]  [<ffffffff811e5523>] vmf_insert_pfn_pmd+0xb3/0x210
> > [  826.187992]  [<ffffffff8124acab>] __dax_pmd_fault+0x3cb/0x610
> > [  826.221337]  [<ffffffffa0769910>] ? ext4_dax_mkwrite+0x20/0x20 [ext4]
> > [  826.259190]  [<ffffffffa0769a4d>] ext4_dax_pmd_fault+0xcd/0x100 [ext4]
> > [  826.293414]  [<ffffffff811b0af7>] handle_mm_fault+0x3b7/0x510
> > [  826.323763]  [<ffffffff81068f98>] __do_page_fault+0x188/0x3f0
> > [  826.358186]  [<ffffffff81069230>] do_page_fault+0x30/0x80
> > [  826.391212]  [<ffffffff8169c148>] page_fault+0x28/0x30
> > [  826.420752] Code: 89 e5 74 09 48 83 c2 01 80 3a 00 75 f7 48 83 c6 01 0f b6 4e ff 48 83
> >  c2 01 84 c9 88 4a ff 75 ed 5d c3 0f 1f 00 55 48 89 e5 eb 04 <84> c0 74 18 48 83 c7 01 0f
> >  b6 47 ff 48 83 c6 01 3a 46 ff 74 eb
> 
> Hmm, a loop in the resource sibling list?
> 
> What does /proc/iomem say?
> 
> Not related to this bug, but lookup_memtype() looks broken for pmd
> mappings as we only check for PAGE_SIZE instead of HPAGE_SIZE.  Which
> will cause problems if we're straddling the end of memory.
> 
> > The full output is large (48 cpus), so I'm going to be lazy and not
> > cut-n-paste it here.
> 
> Thanks for that ;-)

Yea, my first round of testing was broken, sorry about that.

It looks like this test causes the PMD fault handler to be called repeatedly
over and over until you kill the userspace process.  This doesn't happen for
XFS because when using XFS this test doesn't hit PMD faults, only PTE faults.

So, looks like a livelock as far as I can tell.

Still debugging.