On Sun 03-01-16 10:13:06, Dan Williams wrote:

On Wed, Dec 23, 2015 at 11:39 AM, Ross Zwisler
[off-list ref] wrote:

quoted

To properly handle fsync/msync in an efficient way DAX needs to track dirty
pages so it is able to flush them durably to media on demand.

The tracking of dirty pages is done via the radix tree in struct
address_space.  This radix tree is already used by the page writeback
infrastructure for tracking dirty pages associated with an open file, and
it already has support for exceptional (non struct page*) entries.  We
build upon these features to add exceptional entries to the radix tree for
DAX dirty PMD or PTE pages at fault time.

Signed-off-by: Ross Zwisler <redacted>

I'm hitting the following report with the ndctl dax test [1] on
next-20151231.  I bisected it to
 commit 3cb108f941de "dax-add-support-for-fsync-sync-v6".  I'll take a
closer look tomorrow, but in case someone can beat me to it, here's
the back-trace:

------------[ cut here ]------------
kernel BUG at fs/inode.c:497!

I suppose this is the check that mapping->nr_exceptional is zero, isn't it?
Hum, I don't see how that could happen given we call
truncate_inode_pages_final() just before the clear_inode() call which
removes all the exceptional entries from the radix tree.  And there's not
much room for a race during umount... Does the radix tree really contain
any entry or is it an accounting bug?

								Honza

[..]
CPU: 1 PID: 3001 Comm: umount Tainted: G           O    4.4.0-rc7+ #2412
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
task: ffff8800da2a5a00 ti: ffff880307794000 task.ti: ffff880307794000
RIP: 0010:[<ffffffff81280171>]  [<ffffffff81280171>] clear_inode+0x71/0x80
RSP: 0018:ffff880307797d50  EFLAGS: 00010002
RAX: ffff8800da2a5a00 RBX: ffff8800ca2e7328 RCX: ffff8800da2a5a28
RDX: 0000000000000001 RSI: 0000000000000005 RDI: ffff8800ca2e7530
RBP: ffff880307797d60 R08: ffffffff82900ae0 R09: 0000000000000000
R10: ffff8800ca2e7548 R11: 0000000000000000 R12: ffff8800ca2e7530
R13: ffff8800ca2e7328 R14: ffff8800da2e88d0 R15: ffff8800da2e88d0
FS:  00007f2b22f4a880(0000) GS:ffff88031fc40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005648abd933e8 CR3: 000000007f3fc000 CR4: 00000000000006e0
Stack:
ffff8800ca2e7328 ffff8800ca2e7000 ffff880307797d88 ffffffffa01c18af
ffff8800ca2e7328 ffff8800ca2e74d0 ffffffffa01ec740 ffff880307797db0
ffffffff81281038 ffff8800ca2e74c0 ffff880307797e00 ffff8800ca2e7328
Call Trace:
[<ffffffffa01c18af>] xfs_fs_evict_inode+0x5f/0x110 [xfs]
[<ffffffff81281038>] evict+0xb8/0x180
[<ffffffff8128113b>] dispose_list+0x3b/0x50
[<ffffffff81282014>] evict_inodes+0x144/0x170
[<ffffffff8126447f>] generic_shutdown_super+0x3f/0xf0
[<ffffffff81264837>] kill_block_super+0x27/0x70
[<ffffffff81264a53>] deactivate_locked_super+0x43/0x70
[<ffffffff81264e9c>] deactivate_super+0x5c/0x60
[<ffffffff81285aff>] cleanup_mnt+0x3f/0x90
[<ffffffff81285b92>] __cleanup_mnt+0x12/0x20
[<ffffffff810c4f26>] task_work_run+0x76/0x90
[<ffffffff81003e3a>] syscall_return_slowpath+0x20a/0x280
[<ffffffff8192671a>] int_ret_from_sys_call+0x25/0x9f
Code: 48 8d 93 30 03 00 00 48 39 c2 75 23 48 8b 83 d0 00 00 00 a8 20
74 1a a8 40 75 18 48 c7 8
3 d0 00 00 00 60 00 00 00 5b 41 5c 5d c3 <0f> 0b 0f 0b 0f 0b 0f 0b 0f
0b 0f 1f 44 00 00 0f 1f
44 00 00 55
RIP  [<ffffffff81280171>] clear_inode+0x71/0x80
RSP <ffff880307797d50>
---[ end trace 3b1d8898a94a4fc1 ]---

[1]: git://git@github.com:pmem/ndctl.git pending
make TESTS="test/dax.sh" check

-- 
Jan Kara [off-list ref]
SUSE Labs, CR

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