Re: [PATCH v2 00/20] get_user_pages() for dax mappings

[Logan Gunthorpe <logang@deltatee.com>]

Hi Dan,

I've done a bit of digging and here's some more information:

* The crash occurs in ext4_end_io_unwritten when it tries to dereference 
bh->b_assoc_map which is not necessarily NULL.

* That function is called by __dax_pmd_fault, as the argument 
complete_unwritten.

* Looking in __dax_pmd_fault, the bug occurs if we hit either of the 
first two 'goto fallback' lines. (In my case, it's hitting the first one.)

* After the fallback code, it goes back to 'out', then checks '&bh'
for the unwritten flag. But bh hasn't been initialized yet and, on my 
setup, the unwritten flag happens to be set. So, it then calls 
complete_unwritten with a garbage bh and crashes.

If I move the memset(&bh) up in the code, before the goto fallbacks can 
occur, I can fix the crash.  I don't know if this is really the best way 
to fix the problem though.

--

However, unfortunately, fixing the above just uncovered another issue. 
Now the MR de-registration seems to have completed but the task hangs 
when it's trying to munmap the memory. (Stack trace at the end of this 
email.)

It looks like the i_mmap_lock_write is hanging in unlink_file_vma. I'm 
not really sure how to go about debugging this lock issue. If you have 
any steps I can try to get you more information let me know. I'm also 
happy to re-test if you have any other changes you'd like me to try.

Thanks,

Logan


> [ 240.520522] INFO: task client:1997 blocked for more than 120 seconds.
> [ 240.520638] Tainted: G O 4.4.0-rc3+donard2.5+ #87
> [ 240.520741] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 240.520847] client D ffff88047fd14800 0 1997 1912 0x00000004
> [ 240.520856] ffff88026bc7b240 0000000000000000 ffff88026bd38000 ffff88026bd37d30
> [ 240.520861] fffffffeffffffff ffff88026bc7b240 00007f4297513000 ffff880473aba240
> [ 240.520866] ffffffff81422896 ffff880470b34e40 ffffffff814242f1 ffff880476deddc0
> [ 240.520871] Call Trace:
> [ 240.520886] [<ffffffff81422896>] ? schedule+0x6c/0x79
> [ 240.520893] [<ffffffff814242f1>] ? rwsem_down_write_failed+0x285/0x2cb
> [ 240.520903] [<ffffffff8124d833>] ? call_rwsem_down_write_failed+0x13/0x20
> [ 240.520907] [<ffffffff8124d833>] ? call_rwsem_down_write_failed+0x13/0x20
> [ 240.520913] [<ffffffff81423b22>] ? down_write+0x24/0x33
> [ 240.520923] [<ffffffff8110836e>] ? unlink_file_vma+0x28/0x4b
> [ 240.520928] [<ffffffff811033e4>] ? free_pgtables+0x3c/0xba
> [ 240.520933] [<ffffffff81107c15>] ? unmap_region+0xa4/0xc1
> [ 240.520941] [<ffffffff8106c60c>] ? pick_next_task_fair+0x11b/0x347
> [ 240.520947] [<ffffffff8110795f>] ? vma_gap_callbacks_propagate+0x16/0x2c
> [ 240.520951] [<ffffffff81108101>] ? vma_rb_erase+0x161/0x18f
> [ 240.520957] [<ffffffff81109524>] ? do_munmap+0x271/0x2e6
> [ 240.520962] [<ffffffff811095d0>] ? vm_munmap+0x37/0x4f
> [ 240.520967] [<ffffffff81109602>] ? SyS_munmap+0x1a/0x1f
> [ 240.520971] [<ffffffff81424d57>] ? entry_SYSCALL_64_fastpath+0x12/0x6a

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