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Subject[PATCH V2 4/4] blk-mq: fallback to previous nr_hw_queues when updating fails
Date
When we try to increate the nr_hw_queues, we may fail due to
shortage of memory or other reason, then blk_mq_realloc_hw_ctxs stops
and some entries in q->queue_hw_ctx are left with NULL. However,
because queue map has been updated with new nr_hw_queues, some cpus
have been mapped to hw queue which just encounters allocation failure,
thus blk_mq_map_queue could return NULL. This will cause panic in
following blk_mq_map_swqueue.

To fix it, when increase nr_hw_queues fails, fallback to previous
nr_hw_queues and post warning. At the same time, driver's .map_queues
usually use completion irq affinity to map hw and cpu, fallback
nr_hw_queues will cause lack of some cpu's map to hw, so use default
blk_mq_map_queues to do that.

Reported-by: syzbot+83e8cbe702263932d9d4@syzkaller.appspotmail.com
Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
block/blk-mq.c | 27 ++++++++++++++++++++++++---
1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 34f3973..d2ce67a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2540,7 +2540,7 @@ static struct blk_mq_hw_ctx *blk_mq_alloc_and_init_hctx(
static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
struct request_queue *q)
{
- int i, j;
+ int i, j, end;
struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx;

/* protect against switching io scheduler */
@@ -2574,8 +2574,20 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
break;
}
}
+ /*
+ * Increasing nr_hw_queues fails. Free the newly allocated
+ * hctxs and keep the previous q->nr_hw_queues.
+ */
+ if (i != set->nr_hw_queues) {
+ j = q->nr_hw_queues;
+ end = i;
+ } else {
+ j = i;
+ end = q->nr_hw_queues;
+ q->nr_hw_queues = set->nr_hw_queues;
+ }

- for (j = i; j < q->nr_hw_queues; j++) {
+ for (; j < end; j++) {
struct blk_mq_hw_ctx *hctx = hctxs[j];

if (hctx) {
@@ -2587,7 +2599,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,

}
}
- q->nr_hw_queues = i;
mutex_unlock(&q->sysfs_lock);
}

@@ -2972,6 +2983,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
{
struct request_queue *q;
LIST_HEAD(head);
+ int prev_nr_hw_queues;

lockdep_assert_held(&set->tag_list_lock);

@@ -3000,10 +3012,19 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
blk_mq_sysfs_unregister(q);
}

+ prev_nr_hw_queues = set->nr_hw_queues;
set->nr_hw_queues = nr_hw_queues;
blk_mq_update_queue_map(set);
+fallback:
list_for_each_entry(q, &set->tag_list, tag_set_list) {
blk_mq_realloc_hw_ctxs(set, q);
+ if (q->nr_hw_queues != set->nr_hw_queues) {
+ pr_warn("Increasing nr_hw_queues to %d fails, fallback to %d\n",
+ nr_hw_queues, prev_nr_hw_queues);
+ set->nr_hw_queues = prev_nr_hw_queues;
+ blk_mq_map_queues(set);
+ goto fallback;
+ }
blk_mq_map_swqueue(q);
}

--
2.7.4
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