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

[Kirill A. Shutemov]

On Tue, Sep 22, 2015 at 01:05:55PM +1000, Dave Chinner wrote:
> 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.

It's likely we broke some locking ordering rules, but it's not obvious for
me which one.

No lockdep complain? Or it's disabled?

-- 
 Kirill A. Shutemov
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