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Subject[OOPS] elevator private data for REQ_FLUSH
Hello,

Commit
9d5a4e946ce5352f19400b6370f4cd8e72806278
block: skip elevator data initialization for flush requests

Skip elevator initialization for flush requests by passing priv=0 to
blk_alloc_request() in get_request(). As such elv_set_request() is
never called for flush requests.

introduced priv flag, to skip elevator_private data init for FLUSH requests.
This, I guess, lead to NULL pointer deref on my machine in cfq_insert_request,
which requires elevator_private to be set:

1 [ 78.982169] Call Trace:
2 [ 78.982178] [<ffffffff8122d1fe>] cfq_insert_request+0x4e/0x47d
3 [ 78.982184] [<ffffffff8123e139>] ? do_raw_spin_lock+0x6b/0x122
4 [ 78.982189] [<ffffffff81218c4a>] elv_insert+0x212/0x265
5 [ 78.982192] [<ffffffff81218ced>] __elv_add_request+0x50/0x52
6 [ 78.982195] [<ffffffff8121b44a>] flush_plug_list+0xce/0x12f
7 [ 78.982199] [<ffffffff8121b4c0>] __blk_flush_plug+0x15/0x21
8 [ 78.982205] [<ffffffff8146b321>] schedule+0x43e/0xbea
9 [ 78.982211] [<ffffffff8106f8bd>] ? __lock_acquire+0x149d/0x1576
10 [ 78.982215] [<ffffffff8121ba9e>] ? drive_stat_acct+0x1b6/0x1c3
11 [ 78.982218] [<ffffffff8121b92c>] ? drive_stat_acct+0x44/0x1c3
12 [ 78.982223] [<ffffffff8121f641>] ? __make_request+0x268/0x2bf
13 [ 78.982226] [<ffffffff8146bf66>] schedule_timeout+0x35/0x3b8
14 [ 78.982230] [<ffffffff810705ed>] ? mark_held_locks+0x4b/0x6d
15 [ 78.982234] [<ffffffff8146e768>] ? _raw_spin_unlock_irq+0x28/0x56
16 [ 78.982239] [<ffffffff81033bc1>] ? get_parent_ip+0xe/0x3e
17 [ 78.982244] [<ffffffff81471822>] ? sub_preempt_count+0x90/0xa3
18 [ 78.982247] [<ffffffff8146acbb>] wait_for_common+0xc3/0x141
19 [ 78.982251] [<ffffffff8103823a>] ? default_wake_function+0x0/0xf
20 [ 78.982254] [<ffffffff8146ad51>] wait_for_completion+0x18/0x1a
21 [ 78.982258] [<ffffffff8122087b>] blkdev_issue_flush+0xcb/0x11a
22 [ 78.982264] [<ffffffff811a9d65>] ext4_sync_file+0x2b3/0x302
23 [ 78.982268] [<ffffffff81129e25>] vfs_fsync_range+0x55/0x75
24 [ 78.982271] [<ffffffff81129e84>] generic_write_sync+0x3f/0x41
25 [ 78.982278] [<ffffffff810c6c6c>] generic_file_aio_write+0x8c/0xb9
26 [ 78.982281] [<ffffffff811a99a9>] ext4_file_write+0x1dc/0x237
27 [ 78.982285] [<ffffffff8123e139>] ? do_raw_spin_lock+0x6b/0x122
28 [ 78.982288] [<ffffffff811a97cd>] ? ext4_file_write+0x0/0x237
29 [ 78.982292] [<ffffffff81104859>] do_sync_readv_writev+0xb4/0xf9
30 [ 78.982298] [<ffffffff8120af11>] ? security_file_permission+0x1e/0x84
31 [ 78.982302] [<ffffffff8110418e>] ? rw_verify_area+0xab/0xc8
32 [ 78.982305] [<ffffffff81104ac6>] do_readv_writev+0xb8/0x17d
33 [ 78.982309] [<ffffffff81105b5e>] ? fget_light+0x166/0x30b
34 [ 78.982312] [<ffffffff81104bcb>] vfs_writev+0x40/0x42
35 [ 78.982315] [<ffffffff81104e1c>] sys_pwritev+0x55/0x99
36 [ 78.982320] [<ffffffff81474a52>] system_call_fastpath+0x16/0x1b
37

Should we in that case use ELEVATOR_INSERT_FLUSH for REQ_FLUSH | REQ_FUA requests
(like below)?
---
block/elevator.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)
diff --git a/block/elevator.c b/block/elevator.c
index c387d31..b17e577 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -734,6 +734,8 @@ void __elv_add_request(struct request_queue *q, struct request *rq, int where)
q->end_sector = rq_end_sector(rq);
q->boundary_rq = rq;
}
+ } else if (rq->cmd_flags & (REQ_FLUSH | REQ_FUA)) {
+ where = ELEVATOR_INSERT_FLUSH;
} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
where == ELEVATOR_INSERT_SORT)
where = ELEVATOR_INSERT_BACK;



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