Re: [PATCH v4 1/2] blk-mq: Avoid memory reclaim when allocating request map

Xiubo Li <xiubli@xxxxxxxxxx> · Mon, 30 Sep 2019 13:50:04 +0800

On 2019/9/30 13:28, Damien Le Moal wrote:
On 2019/09/29 18:52, xiubli@xxxxxxxxxx wrote:
From: Xiubo Li <xiubli@xxxxxxxxxx>

For some storage drivers, such as the nbd, when there has new socket
connections added, it will update the hardware queue number by calling
blk_mq_update_nr_hw_queues(), in which it will freeze all the queues
first. And then tries to do the hardware queue updating stuff.

But int blk_mq_alloc_rq_map()-->blk_mq_init_tags(), when allocating
memory for tags, it may cause the mm do the memories direct reclaiming,
since the queues has been freezed, so if needs to flush the page cache
to disk, it will stuck in generic_make_request()-->blk_queue_enter() by
waiting the queues to be unfreezed and then cause deadlock here.

Since the memory size requested here is a small one, which will make
it not that easy to happen with a large size, but in theory this could
happen when the OS is running in pressure and out of memory.

Gabriel Krisman Bertazi has hit the similar issue by fixing it in
commit 36e1f3d10786 ("blk-mq: Avoid memory reclaim when remapping
queues"), but might forget this part.

Signed-off-by: Xiubo Li <xiubli@xxxxxxxxxx>
CC: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
  block/blk-mq-tag.c | 5 +++--
  block/blk-mq-tag.h | 5 ++++-
  block/blk-mq.c     | 3 ++-
  3 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 008388e82b5c..04ee0e4c3fa1 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -462,7 +462,8 @@ static struct blk_mq_tags *blk_mq_init_bitmap_tags(struct blk_mq_tags *tags,
  
  struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
  				     unsigned int reserved_tags,
-				     int node, int alloc_policy)
+				     int node, int alloc_policy,
+				     gfp_t flags)
  {
  	struct blk_mq_tags *tags;
  
@@ -471,7 +472,7 @@ struct blk_mq_tags *blk_mq_init_tags(unsigned int total_tags,
  		return NULL;
  	}
  
-	tags = kzalloc_node(sizeof(*tags), GFP_KERNEL, node);
+	tags = kzalloc_node(sizeof(*tags), flags, node);
  	if (!tags)
  		return NULL;
  
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 61deab0b5a5a..296e0bc97126 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -22,7 +22,10 @@ struct blk_mq_tags {
  };
  
  
-extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags, unsigned int reserved_tags, int node, int alloc_policy);
+extern struct blk_mq_tags *blk_mq_init_tags(unsigned int nr_tags,
+					    unsigned int reserved_tags,
+					    int node, int alloc_policy,
+					    gfp_t flags);
  extern void blk_mq_free_tags(struct blk_mq_tags *tags);
  
  extern unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 240416057f28..9c52e4dfe132 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2090,7 +2090,8 @@ struct blk_mq_tags *blk_mq_alloc_rq_map(struct blk_mq_tag_set *set,
  		node = set->numa_node;
  
  	tags = blk_mq_init_tags(nr_tags, reserved_tags, node,
-				BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags));
+				BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags),
+				GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY);
You added the gfp_t argument to blk_mq_init_tags() but you are only using that
argument with a hardcoded value here. So why not simply call kzalloc_node() in
that function with the flags GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY ? That
would avoid needing to add the "gfp_t flags" argument and still fit with your
patch 2 definition of BLK_MQ_GFP_FLAGS.

The blk_mq_init_tags() is defined in another separate file, which I think it means to provide a common way of initializing the tags stuff, and currently in this path it needs GFP_NOIO while in others in future it may not.

Thanks,
BRs


  	if (!tags)
  		return NULL;
  









