Speed up geom_stats_resync in the presence of many devices

The old code had a O(n) loop, where n is the size of /dev/devstat.
Multiply that by another O(n) loop in devstat_mmap for a total of
O(n^2).

This change adds DIOCGMEDIASIZE support to /dev/devstat so userland can
quickly determine the right amount of memory to map, eliminating the
O(n) loop in userland.

This change decreases the time to run "gstat -bI0.001" with 16,384 md
devices from 29.7s to 4.2s.

Also, fix a memory leak first reported as PR 203097.

Sponsored by:	Axcient
Reviewed by:	mav, imp
Differential Revision:	https://reviews.freebsd.org/D28968

(cherry picked from commit ab63da3564e8ab0907f9d8eb565774848ffdadeb)

2 files changed, 38 insertions, 9 deletions

diff --git a/lib/libgeom/geom_stats.c b/lib/libgeom/geom_stats.c
index ebf7868c3c65..450ee491ea1c 100644
--- a/lib/libgeom/geom_stats.c
+++ b/lib/libgeom/geom_stats.c
@@ -32,9 +32,12 @@
  */
 
 #include <sys/types.h>
+#include <sys/ioctl.h>
+#include <sys/disk.h>
 #include <sys/devicestat.h>
 #include <sys/mman.h>
 #include <sys/time.h>
+#include <err.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <paths.h>
@@ -53,7 +56,7 @@ geom_stats_close(void)
 {
 	if (statsfd == -1)
 		return;
-	munmap(statp, npages *pagesize);
+	munmap(statp, npages * pagesize);
 	statp = NULL;
 	close (statsfd);
 	statsfd = -1;
@@ -63,17 +66,22 @@ void
 geom_stats_resync(void)
 {
 	void *p;
+	off_t mediasize;
+	int error;
 
 	if (statsfd == -1)
 		return;
-	for (;;) {
-		p = mmap(statp, (npages + 1) * pagesize,
-		    PROT_READ, MAP_SHARED, statsfd, 0);
-		if (p == MAP_FAILED)
-			break;
-		else
-			statp = p;
-		npages++;
+	error = ioctl(statsfd, DIOCGMEDIASIZE, &mediasize);
+	if (error)
+		err(1, "DIOCGMEDIASIZE(" _PATH_DEV DEVSTAT_DEVICE_NAME ")");
+
+	munmap(statp, npages * pagesize);
+	p = mmap(statp, mediasize, PROT_READ, MAP_SHARED, statsfd, 0);
+	if (p == MAP_FAILED)
+		err(1, "mmap(/dev/devstat):");
+	else {
+		statp = p;
+		npages = mediasize / pagesize;
 	}
 }
 
diff --git a/sys/kern/subr_devstat.c b/sys/kern/subr_devstat.c
index 617e1e439a94..1bbac1839f77 100644
--- a/sys/kern/subr_devstat.c
+++ b/sys/kern/subr_devstat.c
@@ -32,6 +32,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/disk.h>
 #include <sys/kernel.h>
 #include <sys/systm.h>
 #include <sys/bio.h>
@@ -476,10 +477,12 @@ SYSCTL_INT(_kern_devstat, OID_AUTO, version, CTLFLAG_RD,
 
 #define statsperpage (PAGE_SIZE / sizeof(struct devstat))
 
+static d_ioctl_t devstat_ioctl;
 static d_mmap_t devstat_mmap;
 
 static struct cdevsw devstat_cdevsw = {
 	.d_version =	D_VERSION,
+	.d_ioctl =	devstat_ioctl,
 	.d_mmap =	devstat_mmap,
 	.d_name =	"devstat",
 };
@@ -490,10 +493,27 @@ struct statspage {
 	u_int			nfree;
 };
 
+static size_t pagelist_pages = 0;
 static TAILQ_HEAD(, statspage)	pagelist = TAILQ_HEAD_INITIALIZER(pagelist);
 static MALLOC_DEFINE(M_DEVSTAT, "devstat", "Device statistics");
 
 static int
+devstat_ioctl(struct cdev *dev, u_long cmd, caddr_t data, int fflag,
+    struct thread *td)
+{
+	int error = ENOTTY;
+
+	switch (cmd) {
+	case DIOCGMEDIASIZE:
+		error = 0;
+		*(off_t *)data = pagelist_pages * PAGE_SIZE;
+		break;
+	}
+
+	return (error);
+}
+
+static int
 devstat_mmap(struct cdev *dev, vm_ooffset_t offset, vm_paddr_t *paddr,
     int nprot, vm_memattr_t *memattr)
 {
@@ -559,6 +579,7 @@ devstat_alloc(void)
 			 * head but the order on the list determine the
 			 * sequence of the mapping so we can't do that.
 			 */
+			pagelist_pages++;
 			TAILQ_INSERT_TAIL(&pagelist, spp, list);
 		} else
 			break;