[PATCH] nvmet: Reorder fields in 'struct nvmet_ns'

[Christophe JAILLET]

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvmet_ns' from 520 to 512
bytes.

When such a structure is allocated in nvmet_ns_alloc(), because of the way
memory allocation works, when 520 bytes were requested, 1024 bytes were
allocated.

So, on x86_64, this change saves 512 bytes per allocation.

Signed-off-by: Christophe JAILLET <christophe.jaillet at wanadoo.fr>
---
More aggressive grouping could be done to be more future proof, but the
way the struct nvmet_ns is written suggest that some fields should be
kept together. So keep grouping as-is.


Using pahole

Before:
======
struct nvmet_ns {
	struct percpu_ref          ref;                  /*     0    16 */
	struct block_device *      bdev;                 /*    16     8 */
	struct file *              file;                 /*    24     8 */
	bool                       readonly;             /*    32     1 */

	/* XXX 3 bytes hole, try to pack */

	u32                        nsid;                 /*    36     4 */
	u32                        blksize_shift;        /*    40     4 */

	/* XXX 4 bytes hole, try to pack */

	loff_t                     size;                 /*    48     8 */
	u8                         nguid[16];            /*    56    16 */
	/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
	uuid_t                     uuid;                 /*    72    16 */
	u32                        anagrpid;             /*    88     4 */
	bool                       buffered_io;          /*    92     1 */
	bool                       enabled;              /*    93     1 */

	/* XXX 2 bytes hole, try to pack */

	struct nvmet_subsys *      subsys;               /*    96     8 */
	const char  *              device_path;          /*   104     8 */
	struct config_group        device_group;         /*   112   136 */
	/* --- cacheline 3 boundary (192 bytes) was 56 bytes ago --- */
	struct config_group        group;                /*   248   136 */
	/* --- cacheline 6 boundary (384 bytes) --- */
	struct completion          disable_done;         /*   384    96 */
	/* --- cacheline 7 boundary (448 bytes) was 32 bytes ago --- */
	mempool_t *                bvec_pool;            /*   480     8 */
	int                        use_p2pmem;           /*   488     4 */

	/* XXX 4 bytes hole, try to pack */

	struct pci_dev *           p2p_dev;              /*   496     8 */
	int                        pi_type;              /*   504     4 */
	int                        metadata_size;        /*   508     4 */
	/* --- cacheline 8 boundary (512 bytes) --- */
	u8                         csi;                  /*   512     1 */

	/* size: 520, cachelines: 9, members: 23 */
	/* sum members: 500, holes: 4, sum holes: 13 */
	/* padding: 7 */
	/* last cacheline: 8 bytes */
};

After:
=====
struct nvmet_ns {
	struct percpu_ref          ref;                  /*     0    16 */
	struct block_device *      bdev;                 /*    16     8 */
	struct file *              file;                 /*    24     8 */
	bool                       readonly;             /*    32     1 */

	/* XXX 3 bytes hole, try to pack */

	u32                        nsid;                 /*    36     4 */
	u32                        blksize_shift;        /*    40     4 */

	/* XXX 4 bytes hole, try to pack */

	loff_t                     size;                 /*    48     8 */
	u8                         nguid[16];            /*    56    16 */
	/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
	uuid_t                     uuid;                 /*    72    16 */
	u32                        anagrpid;             /*    88     4 */
	bool                       buffered_io;          /*    92     1 */
	bool                       enabled;              /*    93     1 */

	/* XXX 2 bytes hole, try to pack */

	struct nvmet_subsys *      subsys;               /*    96     8 */
	const char  *              device_path;          /*   104     8 */
	struct config_group        device_group;         /*   112   136 */
	/* --- cacheline 3 boundary (192 bytes) was 56 bytes ago --- */
	struct config_group        group;                /*   248   136 */
	/* --- cacheline 6 boundary (384 bytes) --- */
	struct completion          disable_done;         /*   384    96 */
	/* --- cacheline 7 boundary (448 bytes) was 32 bytes ago --- */
	mempool_t *                bvec_pool;            /*   480     8 */
	struct pci_dev *           p2p_dev;              /*   488     8 */
	int                        use_p2pmem;           /*   496     4 */
	int                        pi_type;              /*   500     4 */
	int                        metadata_size;        /*   504     4 */
	u8                         csi;                  /*   508     1 */

	/* size: 512, cachelines: 8, members: 23 */
	/* sum members: 500, holes: 3, sum holes: 9 */
	/* padding: 3 */
};
---
 drivers/nvme/target/nvmet.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvme/target/nvmet.h b/drivers/nvme/target/nvmet.h
index dc60a22646f7..c50146085fb5 100644
--- a/drivers/nvme/target/nvmet.h
+++ b/drivers/nvme/target/nvmet.h
@@ -79,8 +79,8 @@ struct nvmet_ns {
 	struct completion	disable_done;
 	mempool_t		*bvec_pool;
 
-	int			use_p2pmem;
 	struct pci_dev		*p2p_dev;
+	int			use_p2pmem;
 	int			pi_type;
 	int			metadata_size;
 	u8			csi;
-- 
2.34.1