[4.3-rc1, regression] dax: hang on i_mmap_rwsem in generic/075

[Dave Chinner]

Hi folks,

I'm seeing hangs like this when using DAX on XFS on 4.3-rc1 running
xfstests generic/075 (fsx test):

# echo w > /proc/sysrq-trigger
[71628.984872] sysrq: SysRq : Show Blocked State
[71628.985988]   task                        PC stack   pid father
[71628.987635] fsx             D ffff88043fd756c0 12824   520  32636 0x00000000
[71628.989251]  ffff88007f557ba8 0000000000000086 ffff88042eb40580 ffff8803c8bcc180
[71628.990645]  ffff88007f558000 ffff88041d748e80 ffff88041d748e68 ffffffff00000000
[71628.992068]  00000000fffffffe ffff88007f557bc0 ffffffff81d855ca ffff8803c8bcc180
[71628.993639] Call Trace:
[71628.994097]  [<ffffffff81d855ca>] schedule+0x3a/0x90
[71628.994997]  [<ffffffff81d88021>] rwsem_down_write_failed+0x141/0x340
[71628.996197]  [<ffffffff81792e13>] call_rwsem_down_write_failed+0x13/0x20
[71628.997548]  [<ffffffff81d87854>] ? down_write+0x24/0x40
[71628.998502]  [<ffffffff812110b6>] __dax_fault+0x546/0x6c0
[71628.999469]  [<ffffffff814b2900>] ? xfs_get_blocks+0x20/0x20
[71629.000515]  [<ffffffff814bd758>] xfs_filemap_fault+0xc8/0xf0
[71629.001668]  [<ffffffff811a0abd>] __do_fault+0x3d/0x80
[71629.002589]  [<ffffffff811a49da>] handle_mm_fault+0xb8a/0xfd0
[71629.003620]  [<ffffffff81094e3f>] __do_page_fault+0x15f/0x420
[71629.004680]  [<ffffffff810951c3>] trace_do_page_fault+0x43/0x110
[71629.005877]  [<ffffffff8108fd0a>] do_async_page_fault+0x1a/0xa0
[71629.006936]  [<ffffffff81d8b6c8>] async_page_fault+0x28/0x30

__dax_fault() gets stuck on this lock:

(gdb) l *(__dax_fault+0x546)
0xffffffff812110b6 is in __dax_fault (include/linux/fs.h:499).
494
495     int mapping_tagged(struct address_space *mapping, int tag);
496
497     static inline void i_mmap_lock_write(struct address_space *mapping)
498     {
499             down_write(&mapping->i_mmap_rwsem);
500     }
501
502     static inline void i_mmap_unlock_write(struct address_space *mapping)
503     {

This didn't happen on 4.2 + the XFS for-next code that was merged
into 4.3-rc1, so it's come from changes somewhere else in the merge.
I suspect either of these two commits:

46c043e mm: take i_mmap_lock in unmap_mapping_range() for DAX
8431729 dax: fix race between simultaneous faults

as they both modified the i_mmap_lock usage for DAX page faults.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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