Reintroduce IO accounting on zvols on Linux 3.19+

openzfs/zfs@e20cd6f caused us to
lose IO accounting on zvols. When I originally wrote that last year, the
symbols we needed to maintain IO accounting were GPL exported, but
torvalds/linux@394ffa5 provided
suitable symbols for restoring this functionality 4 months later.  We
can call them to restore the IO accounting on Linux 3.19 and later as
well as any older kernels where that patch is backported.

Closes openzfs#3741

Signed-off-by: Richard Yao <ryao@gentoo.org>

config/kernel-generic_io_acct.m4

@@ -0,0 +1,26 @@
+dnl #
+dnl # 3.19 API addition
+dnl #
+dnl # torvalds/linux@394ffa503bc40e32d7f54a9b817264e81ce131b4 allows us to
+dnl # increment iostat counters without generic_make_request().
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_GENERIC_IO_ACCT], [
+	AC_MSG_CHECKING([whether generic IO accounting symbols are avaliable])
+	ZFS_LINUX_TRY_COMPILE_SYMBOL([
+		#include <linux/bio.h>
+
+		void (*generic_start_io_acct_f)(int, unsigned long,
+		    struct hd_struct *) = &generic_start_io_acct;
+		void (*generic_end_io_acct_f)(int, struct hd_struct *,
+		    unsigned long) = &generic_end_io_acct;
+	], [
+		generic_start_io_acct(0, 0, NULL);
+		generic_end_io_acct(0, NULL, 0);
+	], [generic_start_io_acct], [block/bio.c], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_GENERIC_IO_ACCT, 1,
+		    [generic_start_io_acct()/generic_end_io_acct() avaliable])
+	], [
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -94,6 +94,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_KMAP_ATOMIC_ARGS
 	ZFS_AC_KERNEL_FOLLOW_DOWN_ONE
 	ZFS_AC_KERNEL_MAKE_REQUEST_FN
+	ZFS_AC_KERNEL_GENERIC_IO_ACCT
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"

include/linux/blkdev_compat.h

@@ -343,4 +343,9 @@ blk_queue_discard_granularity(struct request_queue *q, unsigned int dg)
  */
 #define	VDEV_HOLDER			((void *)0x2401de7)
 
+#ifndef HAVE_GENERIC_IO_ACCT
+#define generic_start_io_acct(rw, slen, part)
+#define generic_end_io_acct(rw, slen, part)
+#endif
+
 #endif /* _ZFS_BLKDEV_H */

module/zfs/zvol.c

@@ -713,6 +713,10 @@ zvol_request(struct request_queue *q, struct bio *bio)
 	fstrans_cookie_t cookie = spl_fstrans_mark();
 	uint64_t offset = BIO_BI_SECTOR(bio);
 	unsigned int sectors = bio_sectors(bio);
+	int rw = bio_data_dir(bio);
+#ifdef HAVE_GENERIC_IO_ACCT
+	unsigned long start = jiffies;
+#endif
 	int error = 0;
 
 	if (bio_has_data(bio) && offset + sectors >
@@ -723,25 +727,29 @@ zvol_request(struct request_queue *q, struct bio *bio)
 		    (long long unsigned)offset,
 		    (long unsigned)sectors);
 		error = SET_ERROR(EIO);
-		goto out;
+		goto out1;
 	}
 
-	if (bio_data_dir(bio) == WRITE) {
+	generic_start_io_acct(rw, sectors, zv->zv_disk->part0);
+
+	if (rw == WRITE) {
 		if (unlikely(zv->zv_flags & ZVOL_RDONLY)) {
 			error = SET_ERROR(EROFS);
-			goto out;
+			goto out2;
 		}
 
 		if (bio->bi_rw & VDEV_REQ_DISCARD) {
 			error = zvol_discard(bio);
-			goto out;
+			goto out2;
 		}
 
 		error = zvol_write(bio);
 	} else
 		error = zvol_read(bio);
 
-out:
+out2:
+	generic_end_io_acct(rw, zv->zv_disk->part0, start);
+out1:
 	bio_endio(bio, -error);
 	spl_fstrans_unmark(cookie);
 #ifdef HAVE_MAKE_REQUEST_FN_RET_INT