Re: [RFC v3] [PATCH 0/18] DAX page fault locking

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

On Mon, Apr 18, 2016 at 11:35:23PM +0200, Jan Kara wrote:
> Hello,
> 
> this is my third attempt at DAX page fault locking rewrite. The patch set has
> passed xfstests both with and without DAX mount option on ext4 and xfs for
> me and also additional page fault beating using the new page fault stress
> tests I have added to xfstests. So I'd be grateful if you guys could have a
> closer look at the patches so that they can be merged. Thanks.
> 
> Changes since v2:
> - lot of additional ext4 fixes and cleanups
> - make PMD page faults depend on CONFIG_BROKEN instead of #if 0
> - fixed page reference leak when replacing hole page with a pfn
> - added some reviewed-by tags
> - rebased on top of current Linus' tree
> 
> Changes since v1:
> - handle wakeups of exclusive waiters properly
> - fix cow fault races
> - other minor stuff
> 
> General description
> 
> The basic idea is that we use a bit in an exceptional radix tree entry as
> a lock bit and use it similarly to how page lock is used for normal faults.
> That way we fix races between hole instantiation and read faults of the
> same index. For now I have disabled PMD faults since there the issues with
> page fault locking are even worse. Now that Matthew's multi-order radix tree
> has landed, I can have a look into using that for proper locking of PMD faults
> but first I want normal pages sorted out.
> 
> In the end I have decided to implement the bit locking directly in the DAX
> code. Originally I was thinking we could provide something generic directly
> in the radix tree code but the functions DAX needs are rather specific.
> Maybe someone else will have a good idea how to distill some generally useful
> functions out of what I've implemented for DAX but for now I didn't bother
> with that.
> 
> 								Honza

Hey Jan,

Another hit in testing, which may or may not be related to the last one.  The
BUG is a few lines off from the previous report:
	kernel BUG at mm/workingset.c:423!
vs
	kernel BUG at mm/workingset.c:435!

I've been able to consistently hit this one using DAX + ext4 with generic/086.
For some reason generic/086 always passes when run by itself, but fails
consistently if you run it after a set of other tests.  Here is a relatively
fast set that reproduces it:

  # ./check generic/070 generic/071 generic/072 generic/073 generic/074 generic/075 generic/076 generic/077 generic/078 generic/079 generic/080 generic/082 generic/083 generic/084 generic/085 generic/086
  FSTYP         -- ext4
  PLATFORM      -- Linux/x86_64 lorwyn 4.6.0-rc5+
  MKFS_OPTIONS  -- /dev/pmem0p2
  MOUNT_OPTIONS -- -o dax -o context=system_u:object_r:nfs_t:s0 /dev/pmem0p2 /mnt/xfstests_scratch
  
  generic/070 1s ... 2s
  generic/071 1s ... 0s
  generic/072 3s ... 4s
  generic/073 1s ... [not run] Cannot run tests with DAX on dmflakey devices
  generic/074 91s ... 85s
  generic/075 1s ... 2s
  generic/076 1s ... 2s
  generic/077 3s ... 2s
  generic/078 1s ... 1s
  generic/079 0s ... 1s
  generic/080 3s ... 2s
  generic/082 0s ... 1s
  generic/083 1s ... 4s
  generic/084 6s ... 6s
  generic/085 5s ... 4s
  generic/086 1s ..../check: line 519: 19233 Segmentation fault      ./$seq > $tmp.rawout 2>&1
   [failed, exit status 139] - output mismatch (see /root/xfstests/results//generic/086.out.bad)
      --- tests/generic/086.out	2015-10-02 10:19:36.799795846 -0600
      +++ /root/xfstests/results//generic/086.out.bad	2016-05-06 13:38:17.682976273 -0600
      @@ -1,14 +1 @@
       QA output created by 086
      -00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      -*
      -00001000  aa aa aa aa aa aa aa aa  aa aa aa aa aa aa aa aa  |................|
      -*
      -00001800  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      -*
      ...
      (Run 'diff -u tests/generic/086.out /root/xfstests/results//generic/086.out.bad'  to see the entire diff)

And the kernel log, passed through kasan_symbolize.py:

 ------------[ cut here ]------------
 kernel BUG at mm/workingset.c:435!
 invalid opcode: 0000 [#1] SMP
 Modules linked in: nd_pmem nd_btt nd_e820 libnvdimm
 CPU: 1 PID: 19233 Comm: 086 Not tainted 4.6.0-rc5+ #2
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150714_191134- 04/01/2014
 task: ffff8800ba88ca40 ti: ffff88040bde4000 task.ti: ffff88040bde4000
 RIP: 0010:[<ffffffff81207b9d>]  [<ffffffff81207b9d>] shadow_lru_isolate+0x18d/0x1a0
 RSP: 0018:ffff88040bde7be8  EFLAGS: 00010006
 RAX: 0000000000000100 RBX: ffff8801f2c91dc0 RCX: ffff8801f2c91fd0
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff880410b2ac00
 RBP: ffff88040bde7c10 R08: 0000000000000004 R09: 0000000000000000
 R10: ffff8801fa31b038 R11: 0000000000000080 R12: ffff880410b2ac00
 R13: ffff8801fa31b020 R14: ffff8801fa31b008 R15: ffff880410b2ac48
 FS:  00007f67ccee8700(0000) GS:ffff88041a200000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007f67ccf00000 CR3: 000000040e434000 CR4: 00000000000006e0
 Stack:
  ffff880410b2ac00 ffff880410b2ac48 ffff88040bde7d18 ffff8801fa2cfb78
  ffff8801f2c91dc0 ffff88040bde7c60 ffffffff81206c7f 0000000000000000
  0000000000000000 ffffffff81207a10 ffff88040bde7d10 0000000000000000
 Call Trace:
  [<ffffffff81206c7f>] __list_lru_walk_one.isra.3+0x9f/0x150 mm/list_lru.c:223
  [<ffffffff81206d53>] list_lru_walk_one+0x23/0x30 mm/list_lru.c:263
  [<     inline     >] list_lru_shrink_walk include/linux/list_lru.h:170
  [<ffffffff81207bea>] scan_shadow_nodes+0x3a/0x50 mm/workingset.c:457
  [<     inline     >] do_shrink_slab mm/vmscan.c:344
  [<ffffffff811ea37e>] shrink_slab.part.40+0x1fe/0x420 mm/vmscan.c:442
  [<ffffffff811ea5c9>] shrink_slab+0x29/0x30 mm/vmscan.c:406
  [<ffffffff811ec831>] drop_slab_node+0x31/0x60 mm/vmscan.c:460
  [<ffffffff811ec89f>] drop_slab+0x3f/0x70 mm/vmscan.c:471
  [<ffffffff812d8c39>] drop_caches_sysctl_handler+0x69/0xb0 fs/drop_caches.c:58
  [<ffffffff812f2937>] proc_sys_call_handler+0xe7/0x100 fs/proc/proc_sysctl.c:543
  [<ffffffff812f2964>] proc_sys_write+0x14/0x20 fs/proc/proc_sysctl.c:561
  [<ffffffff81269aa7>] __vfs_write+0x37/0x120 fs/read_write.c:529
  [<ffffffff8126a3fc>] vfs_write+0xac/0x1a0 fs/read_write.c:578
  [<     inline     >] SYSC_write fs/read_write.c:625
  [<ffffffff8126b8d8>] SyS_write+0x58/0xd0 fs/read_write.c:617
  [<ffffffff81a92a3c>] entry_SYSCALL_64_fastpath+0x1f/0xbd arch/x86/entry/entry_64.S:207
 Code: fa 66 66 90 66 66 90 e8 52 5c ef ff 4c 89 e7 e8 ba a1 88 00 89 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3 0f 0b 0f 0b 0f 0b 0f 0b 0f 0b <0f> 0b 0f 0b 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 66 66
 RIP  [<ffffffff81207b9d>] shadow_lru_isolate+0x18d/0x1a0 mm/workingset.c:422
  RSP <ffff88040bde7be8>

Same setup as last time, a pair of PMEM ramdisks.  This was created with the
same working tree:

https://git.kernel.org/cgit/linux/kernel/git/zwisler/linux.git/log/?h=jan_testing

Thanks!
- Ross

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