RE: [PATCH] nvme: use correct upper limit for tag in nvme_handle_cqe()

From: Tianxianting
Date: Fri Sep 18 2020 - 07:07:13 EST

I am very sorry,
Please ignore this patch, the code need to rewrite.

-----Original Message-----
From: tianxianting (RD)
Sent: Friday, September 18, 2020 3:45 PM
To: kbusch@xxxxxxxxxx; axboe@xxxxxx; hch@xxxxxx; sagi@xxxxxxxxxxx
Cc: linux-nvme@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; tianxianting (RD) <tian.xianting@xxxxxxx>
Subject: [PATCH] nvme: use correct upper limit for tag in nvme_handle_cqe()

We met a crash issue when hot-insert a nvme device, blk_mq_tag_to_rq() returned null(req=null), then crash happened in nvme_end_request():
req = blk_mq_tag_to_rq();
struct nvme_request *rq = nvme_req(req); //rq = req + 1
rq->result = result; <==crash here!!!

[ 1124.256246] nvme nvme5: pci function 0000:e1:00.0
[ 1124.256323] nvme 0000:e1:00.0: enabling device (0000 -> 0002)
[ 1125.720859] nvme nvme5: 96/0/0 default/read/poll queues
[ 1125.732483] nvme5n1: p1 p2 p3
[ 1125.788049] BUG: unable to handle kernel NULL pointer dereference at 0000000000000130
[ 1125.788054] PGD 0 P4D 0
[ 1125.788057] Oops: 0002 [#1] SMP NOPTI
[ 1125.788059] CPU: 50 PID: 0 Comm: swapper/50 Kdump: loaded Tainted: G ------- -t - 4.18.0-147.el8.x86_64 #1
[ 1125.788065] RIP: 0010:nvme_irq+0xe8/0x240 [nvme]
[ 1125.788068] RSP: 0018:ffff916b8ec83ed0 EFLAGS: 00010813
[ 1125.788069] RAX: 0000000000000000 RBX: ffff918ae9211b00 RCX: 0000000000000000
[ 1125.788070] RDX: 000000000000400b RSI: 0000000000000000 RDI: 0000000000000000
[ 1125.788071] RBP: ffff918ae8870000 R08: 0000000000000004 R09: ffff918ae8870000
[ 1125.788072] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 1125.788073] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001
[ 1125.788075] FS: 0000000000000000(0000) GS:ffff916b8ec80000(0000) knlGS:0000000000000000
[ 1125.788075] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1125.788076] CR2: 0000000000000130 CR3: 0000001768f00000 CR4: 0000000000340ee0
[ 1125.788077] Call Trace:
[ 1125.788080] <IRQ>
[ 1125.788085] __handle_irq_event_percpu+0x40/0x180
[ 1125.788087] handle_irq_event_percpu+0x30/0x80
[ 1125.788089] handle_irq_event+0x36/0x53
[ 1125.788090] handle_edge_irq+0x82/0x190
[ 1125.788094] handle_irq+0xbf/0x100
[ 1125.788098] do_IRQ+0x49/0xd0
[ 1125.788100] common_interrupt+0xf/0xf

The analysis of the possible cause of above crash as below.

According to our test, in nvme_pci_enable(), 'dev->q_depth' is set to 1024, in nvme_create_io_queues()->nvme_alloc_queue(), 'nvmeq->q_depth' is set to 'dev->q_depth'. In nvme_dev_add(), 'dev->tagset.queue_depth' is set to 1023:
dev->tagset.queue_depth = min_t(unsigned int, dev->q_depth, BLK_MQ_MAX_DEPTH) - 1;
//why -1?? first involved by commit a4aea562 In nvme_alloc_queue(), 'nvmeq->q_depth' is set to dev->q_depth'.
So we got below values for mutiple depth:
dev->q_depth = 1024
dev->tagset.queue_depth = 1023
nvmeq->q_depth = 1024

In blk_mq_alloc_rqs(), it allocated 1023(dev->tagset.queue_depth) rqs for one hw queue. In nvme_alloc_queue(), it allocated 1024(nvmeq->q_depth) cqes for nvmeq->cqes[]. When process cqe in nvme_handle_cqe(), it get it by:
struct nvme_completion *cqe = &nvmeq->cqes[idx];

We also added below prints in nvme_handle_cqe() and blk_mq_tag_to_rq():
static inline void nvme_handle_cqe(struct nvme_queue *nvmeq, u16 idx)
{
volatile struct nvme_completion *cqe = &nvmeq->cqes[idx];
struct request *req;

//debug print
dev_warn(nvmeq->dev->ctrl.device,
"command_id %d completed on queue %d, nvmeq q_depth %d, nvme tagset q_depth %d\n",
cqe->command_id, le16_to_cpu(cqe->sq_id),
nvmeq->q_depth, nvmeq->dev->tagset.queue_depth);

if (unlikely(cqe->command_id >= nvmeq->q_depth)) {
dev_warn(nvmeq->dev->ctrl.device,
"invalid id %d completed on queue %d\n",
cqe->command_id, le16_to_cpu(cqe->sq_id));
return;
}

... ...

req = blk_mq_tag_to_rq(nvme_queue_tagset(nvmeq), cqe->command_id);
... ...
}

struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
{
//debug print
printk("tag, nr_tags:%d %d\n", tag, tags->nr_tags);
if (tag < tags->nr_tags) {
prefetch(tags->rqs[tag]);
return tags->rqs[tag];
}

return NULL;
}

According to the output, we know the max tag number(nr_tags) is 1023, but
nvmeq->q_depth is 1024 and nvmeq->cqes[] has 1024 cqes. So if
nvmeq->command_id(tag)
is 1023, the judgement "if (unlikely(cqe->command_id >= nvmeq->q_depth))"
in nvme_handle_cqe() is useless, we will get a null pointer, which is returned by blk_mq_tag_to_rq():
[ 16.649973] nvme nvme0: command_id 968 completed on queue 13, nvmeq q_depth 1024, nvme tagset q_depth 1023
[ 16.649974] tag, nr_tags:968 1023

This patch is to make command_id match to its correct upper limit 'nvmeq->dev->tagset.queue_depth', not nvmeq->q_depth. So even if we got 1023 of command_id, we can avoid a null pointer deference.

Signed-off-by: Xianting Tian <tian.xianting@xxxxxxx>
---
drivers/nvme/host/pci.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c index 899d2f4d7..c681e26d0 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -940,7 +940,7 @@ static inline void nvme_handle_cqe(struct nvme_queue *nvmeq, u16 idx)
struct nvme_completion *cqe = &nvmeq->cqes[idx];
struct request *req;

- if (unlikely(cqe->command_id >= nvmeq->q_depth)) {
+ if (unlikely(cqe->command_id >= nvmeq->dev->tagset.queue_depth)) {
dev_warn(nvmeq->dev->ctrl.device,
"invalid id %d completed on queue %d\n",
cqe->command_id, le16_to_cpu(cqe->sq_id));
--
2.17.1