Re: [RFC PATCH] blk-mq: Avoid memory reclaim when allocating request map

[Ming Lei <ming.lei@redhat.com>]

On Sun, Sep 15, 2019 at 05:56:56PM +0530, xiubli@redhat.com wrote:
> From: Xiubo Li <xiubli@redhat.com>
> 
> 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@redhat.com>
> CC: Gabriel Krisman Bertazi <krisman@linux.vnet.ibm.com>
> ---
>  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);

Now, there are three uses on 'GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY',
and code gets cleaner if you make it as one const variable in
blk_mq_alloc_rq_map.

Otherwise, looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>


thanks,
Ming