[PATCH 11/19] blk-mq: Reduce blk_mq_hw_ctx size

[Bart Van Assche]

Since the srcu structure is rather large (184 bytes on an x86-64
system), only allocate it if needed.

Reported-by: Ming Lei <ming....@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanass...@sandisk.com>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Hannes Reinecke <h...@suse.com>
Cc: Omar Sandoval <osan...@fb.com>
Cc: Ming Lei <ming....@redhat.com>
---
 block/blk-mq.c         | 16 +++++++++++++++-
 include/linux/blk-mq.h |  5 +++--
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index bc96e6007e53..b8092786d42f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2227,6 +2227,20 @@ struct request_queue *blk_mq_init_queue(struct 
blk_mq_tag_set *set)
 }
 EXPORT_SYMBOL(blk_mq_init_queue);
 
+static int blk_mq_hw_ctx_size(struct request_queue *q)
+{
+       int hw_ctx_size = sizeof(struct blk_mq_hw_ctx);
+
+       BUILD_BUG_ON(ALIGN(offsetof(struct blk_mq_hw_ctx, queue_rq_srcu),
+                          __alignof__(struct blk_mq_hw_ctx)) !=
+                    sizeof(struct blk_mq_hw_ctx));
+
+       if (tag_set->flags & BLK_MQ_F_BLOCKING)
+               hw_ctx_size += sizeof(struct srcu_struct);
+
+       return hw_ctx_size;
+}
+
 static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
                                                struct request_queue *q)
 {
@@ -2241,7 +2255,7 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set 
*set,
                        continue;
 
                node = blk_mq_hw_queue_to_node(q->mq_map, i);
-               hctxs[i] = kzalloc_node(sizeof(struct blk_mq_hw_ctx),
+               hctxs[i] = kzalloc_node(blk_mq_hw_ctx_size(q),
                                        GFP_KERNEL, node);
                if (!hctxs[i])
                        break;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index c63d1d98404a..c589b81be491 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -39,8 +39,6 @@ struct blk_mq_hw_ctx {
        struct blk_mq_tags      *tags;
        struct blk_mq_tags      *sched_tags;
 
-       struct srcu_struct      queue_rq_srcu[1];
-
        unsigned long           queued;
        unsigned long           run;
 #define BLK_MQ_MAX_DISPATCH_ORDER      7
@@ -62,6 +60,9 @@ struct blk_mq_hw_ctx {
        struct dentry           *debugfs_dir;
        struct dentry           *sched_debugfs_dir;
 #endif
+
+       /* Must be the last member - see also blk_mq_hw_ctx_size(). */
+       struct srcu_struct      queue_rq_srcu[0];
 };
 
 struct blk_mq_tag_set {
-- 
2.12.2