When some buggy driver doesn't set its queue->limits.discard_granularity
(e.g. current loop device driver), discard at LBA 0 on such device will
trigger a kernel BUG() panic from block/blk-mq.c:563.
[ 955.565006][ C39] ------------[ cut here ]------------
[ 955.559660][ C39] invalid opcode: 0000 [#1] SMP NOPTI
[ 955.622171][ C39] CPU: 39 PID: 248 Comm: ksoftirqd/39 Tainted: G E 5.8.0-default+ #40
[ 955.622171][ C39] Hardware name: Lenovo ThinkSystem SR650 -[7X05CTO1WW]-/-[7X05CTO1WW]-, BIOS -[IVE160M-2.70]- 07/17/2020
[ 955.622175][ C39] RIP: 0010:blk_mq_end_request+0x107/0x110
[ 955.622177][ C39] Code: 48 8b 03 e9 59 ff ff ff 48 89 df 5b 5d 41 5c e9 9f ed ff ff 48 8b 35 98 3c f4 00 48 83 c7 10 48 83 c6 19 e8 cb 56 c9 ff eb cb <0f> 0b 0f 1f 80 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 41 56 41 54
[ 955.622179][ C39] RSP: 0018:ffffb1288701fe28 EFLAGS: 00010202
[ 955.749277][ C39] RAX: 0000000000000001 RBX: ffff956fffba5080 RCX: 0000000000004003
[ 955.749278][ C39] RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000000
[ 955.749279][ C39] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[ 955.749279][ C39] R10: ffffb1288701fd28 R11: 0000000000000001 R12: ffffffffa8e05160
[ 955.749280][ C39] R13: 0000000000000004 R14: 0000000000000004 R15: ffffffffa7ad3a1e
[ 955.749281][ C39] FS: 0000000000000000(0000) GS:ffff95bfbda00000(0000) knlGS:0000000000000000
[ 955.749282][ C39] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 955.749282][ C39] CR2: 00007f6f0ef766a8 CR3: 0000005a37012002 CR4: 00000000007606e0
[ 955.749283][ C39] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 955.749284][ C39] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 955.749284][ C39] PKRU: 55555554
[ 955.749285][ C39] Call Trace:
[ 955.749290][ C39] blk_done_softirq+0x99/0xc0
[ 957.550669][ C39] __do_softirq+0xd3/0x45f
[ 957.550677][ C39] ? smpboot_thread_fn+0x2f/0x1e0
[ 957.550679][ C39] ? smpboot_thread_fn+0x74/0x1e0
[ 957.550680][ C39] ? smpboot_thread_fn+0x14e/0x1e0
[ 957.550684][ C39] run_ksoftirqd+0x30/0x60
[ 957.550687][ C39] smpboot_thread_fn+0x149/0x1e0
[ 957.886225][ C39] ? sort_range+0x20/0x20
[ 957.886226][ C39] kthread+0x137/0x160
[ 957.886228][ C39] ? kthread_park+0x90/0x90
[ 957.886231][ C39] ret_from_fork+0x22/0x30
[ 959.117120][ C39] ---[ end trace 3dacdac97e2ed164 ]---
This is the procedure to reproduce the panic,
# modprobe scsi_debug delay=0 dev_size_mb=2048 max_queue=1
# losetup -f /dev/nvme0n1 --direct-io=on
# blkdiscard /dev/loop0 -o 0 -l 0x200
This is how the BUG() panic triggered by __blkdev_issue_discard(),
- For a NVMe SSD backing loop device, the driver does not initialize
its queue->limits.discard_granularity and leaves it to 0.
- When discard on LBA 0 of the loop device, __blkdev_issue_discard()
is called before loop device driver code.
- Inside __blkdev_issue_discard(), when calculating value of
granularity_aligned_lba by
granularity_aligned_lba = round_up(sector_mapped,
q->limits.discard_granularity >> SECTOR_SHIFT);
because sector_mapped is 0 (at LBA 0 and no partition offset), and
q->limits.discard_granularity is 0 (by the buggy loop driver), the
calculated granularity_aligned_lba is 0.
- The inline function bio_aligned_discard_max_sectors() is defined as
return round_down(UINT_MAX, q->limits.discard_granularity) >>
SECTOR_SHIFT;
when q->limits.discard_granularity is 0 from loop device driver, the
above calculation returns value 0.
- Now granularity_aligned_lba and sctor_mapped are 0, req_sectors is
calculated by the following lines in __blkdev_issue_discard(),
if (granularity_aligned_lba == sector_mapped)
req_sects = min_t(sector_t, nr_sects,
bio_aligned_discard_max_sectors(q));
because bio_aligned_discard_max_sectors(q) returns 0, req_sects is
calculated as 0.
- Now a discard bio is mistakenly initialized as a 0 byte bio by,
bio->bi_iter.bi_size = req_sects << 9;
and sent to loop device driver.
- This discard request is handled by loop device driver by following
code path,
loop_handle_cmd => do_req_filebacked => lo_fallocate =>
file->f_op->fallocate => blkdev_fallocate => blkdev_issue_zeroout =>
__blkdev_issue_write_zeroes
- Inside __blkdev_issue_write_zeroes(), a 0 byte length discard bio is
composed and sent to the backing device of the loop device.
- In the I/O completion code path, in my case it is,
blk_done_softirq => nrq->q->mq_ops->complete => nvme_pci_complete_rq
=> nvme_complete_rq => blk_mq_end_request
inside blk_mq_end_request(), blk_update_request() is called and due to
req->bio is NULL in previous step, blk_update_request() returns false
then the BUG() panic in blk_mq_end_request() is triggered.
Although the above panic can be fixed in loop device driver, the generic
__blkdev_issue_discard() should also be fixed to tolerate the incorrect
0 value from queue->limits.discard_granularity, in case some other buggy
driver makes such mistake again.
This patch checks whether q->limits.discard_granularity is 0 in
__blkdev_issue_discard() and bio_aligned_discard_max_sectors(). If it is
0 from some buggy driver queue, prints a warning oops information and
set queue_logical_block_size(q) to a local variable discard_granularity.
This local variable is used in round_up() and round_down() calculation,
now req_sects won't be 0 and no empty discard request is generated.
Fixes: 9b15d109a6b2 ("block: improve discard bio alignment in __blkdev_issue_discard()")
Fixes: c52abf563049 ("loop: Better discard support for block devices")
Reported-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Coly Li <colyli@xxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Xiao Ni <xni@xxxxxxxxxx>
Cc: Bart Van Assche <bvanassche@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Enzo Matsumiya <ematsumiya@xxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Evan Green <evgreen@xxxxxxxxxxxx>
---
block/blk-lib.c | 8 +++++++-
block/blk.h | 9 +++++++--
2 files changed, 14 insertions(+), 3 deletions(-)
diff --git a/block/blk-lib.c b/block/blk-lib.c
index 019e09bb9c0e..3017e4cba923 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -30,6 +30,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
struct bio *bio = *biop;
unsigned int op;
sector_t bs_mask, part_offset = 0;
+ sector_t discard_granularity;
if (!q)
return -ENXIO;
@@ -54,6 +55,11 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
if (!nr_sects)
return -EINVAL;
+ discard_granularity = q->limits.discard_granularity;
+ /* In case some buggy driver does not set limits.discard_granularity */
+ if (WARN_ON_ONCE(discard_granularity == 0))
+ discard_granularity = queue_logical_block_size(q);
+
/* In case the discard request is in a partition */
if (bdev->bd_partno)
part_offset = bdev->bd_part->start_sect;
@@ -63,7 +69,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
sector_t sector_mapped = sector + part_offset;
granularity_aligned_lba = round_up(sector_mapped,
- q->limits.discard_granularity >> SECTOR_SHIFT);
+ discard_granularity >> SECTOR_SHIFT);
/*
* Check whether the discard bio starts at a discard_granularity
diff --git a/block/blk.h b/block/blk.h
index 49e2928a1632..b52aa7049fb0 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -274,8 +274,13 @@ static inline unsigned int bio_allowed_max_sectors(struct request_queue *q)
static inline unsigned int bio_aligned_discard_max_sectors(
struct request_queue *q)
{
- return round_down(UINT_MAX, q->limits.discard_granularity) >>
- SECTOR_SHIFT;
+ sector_t discard_granularity = q->limits.discard_granularity;
+
+ /* In case some buggy driver doesn't set >limits.discard_granularity */
+ if (WARN_ON_ONCE(discard_granularity == 0))
+ discard_granularity = queue_logical_block_size(q);
+
+ return round_down(UINT_MAX, discard_granularity) >> SECTOR_SHIFT;
}
/*
--
2.26.2
