block: cache inode size in bdev

Reading the inode size brings in a new cacheline for IO submit, and
it's in the hot path being checked for every single IO. When doing
millions of IOs per core per second, this is noticeable overhead.

Cache the nr_sectors in the bdev itself.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -58,6 +58,7 @@ void set_capacity(struct gendisk *disk, sector_t sectors)
 
 	spin_lock(&bdev->bd_size_lock);
 	i_size_write(bdev->bd_inode, (loff_t)sectors << SECTOR_SHIFT);
+	bdev->bd_nr_sectors = sectors;
 	spin_unlock(&bdev->bd_size_lock);
 }
 EXPORT_SYMBOL(set_capacity);

block/partitions/core.c

@@ -91,6 +91,7 @@ static void bdev_set_nr_sectors(struct block_device *bdev, sector_t sectors)
 {
 	spin_lock(&bdev->bd_size_lock);
 	i_size_write(bdev->bd_inode, (loff_t)sectors << SECTOR_SHIFT);
+	bdev->bd_nr_sectors = sectors;
 	spin_unlock(&bdev->bd_size_lock);
 }
 

include/linux/blk_types.h

@@ -39,6 +39,7 @@ struct bio_crypt_ctx;
 
 struct block_device {
 	sector_t		bd_start_sect;
+	sector_t		bd_nr_sectors;
 	struct disk_stats __percpu *bd_stats;
 	unsigned long		bd_stamp;
 	bool			bd_read_only;	/* read-only policy */

include/linux/genhd.h

@@ -236,14 +236,14 @@ static inline sector_t get_start_sect(struct block_device *bdev)
 	return bdev->bd_start_sect;
 }
 
-static inline loff_t bdev_nr_bytes(struct block_device *bdev)
+static inline sector_t bdev_nr_sectors(struct block_device *bdev)
 {
-	return i_size_read(bdev->bd_inode);
+	return bdev->bd_nr_sectors;
 }
 
-static inline sector_t bdev_nr_sectors(struct block_device *bdev)
+static inline loff_t bdev_nr_bytes(struct block_device *bdev)
 {
-	return bdev_nr_bytes(bdev) >> SECTOR_SHIFT;
+	return bdev_nr_sectors(bdev) << SECTOR_SHIFT;
 }
 
 static inline sector_t get_capacity(struct gendisk *disk)