Combine zio caches if possible

This deduplicates 2 sets of caches which use the same allocation size.

Memory savings fluctuate a lot, one sample result is FreeBSD running
"make buildworld" saving ~180MB RAM in reduced page count associated
with zio caches.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes openzfs#11877

module/zfs/zio.c

@@ -205,6 +205,19 @@ zio_init(void)
 
 		if (align != 0) {
 			char name[36];
+			if (cflags == data_cflags) {
+				/*
+				 * Resulting kmem caches would be identical.
+				 * Save memory by creating only one.
+				 */
+				(void) snprintf(name, sizeof (name),
+				    "zio_buf_comb_%lu", (ulong_t)size);
+				zio_buf_cache[c] = kmem_cache_create(name,
+				    size, align, NULL, NULL, NULL, NULL, NULL,
+				    cflags);
+				zio_data_buf_cache[c] = zio_buf_cache[c];
+				continue;
+			}
 			(void) snprintf(name, sizeof (name), "zio_buf_%lu",
 			    (ulong_t)size);
 			zio_buf_cache[c] = kmem_cache_create(name, size,
@@ -235,37 +248,50 @@ zio_init(void)
 void
 zio_fini(void)
 {
-	size_t c;
-	kmem_cache_t *last_cache = NULL;
-	kmem_cache_t *last_data_cache = NULL;
+	size_t n = SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT;
 
-	for (c = 0; c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT; c++) {
-#ifdef _ILP32
-		/*
-		 * Cache size limited to 1M on 32-bit platforms until ARC
-		 * buffers no longer require virtual address space.
-		 */
-		if (((c + 1) << SPA_MINBLOCKSHIFT) > zfs_max_recordsize)
-			break;
-#endif
 #if defined(ZFS_DEBUG) && !defined(_KERNEL)
-		if (zio_buf_cache_allocs[c] != zio_buf_cache_frees[c])
+	for (size_t i = 0; i < n; i++) {
+		if (zio_buf_cache_allocs[i] != zio_buf_cache_frees[i])
 			(void) printf("zio_fini: [%d] %llu != %llu\n",
-			    (int)((c + 1) << SPA_MINBLOCKSHIFT),
-			    (long long unsigned)zio_buf_cache_allocs[c],
-			    (long long unsigned)zio_buf_cache_frees[c]);
+			    (int)((i + 1) << SPA_MINBLOCKSHIFT),
+			    (long long unsigned)zio_buf_cache_allocs[i],
+			    (long long unsigned)zio_buf_cache_frees[i]);
+	}
 #endif
-		if (zio_buf_cache[c] != last_cache) {
-			last_cache = zio_buf_cache[c];
-			kmem_cache_destroy(zio_buf_cache[c]);
+
+	/*
+	 * The same kmem cache can show up multiple times in both zio_buf_cache
+	 * and zio_data_buf_cache. Do a wasteful but trivially correct scan to
+	 * sort it out.
+	 */
+	for (size_t i = 0; i < n; i++) {
+		kmem_cache_t *cache = zio_buf_cache[i];
+		if (cache == NULL)
+			continue;
+		for (size_t j = i; j < n; j++) {
+			if (cache == zio_buf_cache[j])
+				zio_buf_cache[j] = NULL;
+			if (cache == zio_data_buf_cache[j])
+				zio_data_buf_cache[j] = NULL;
 		}
-		zio_buf_cache[c] = NULL;
+		kmem_cache_destroy(cache);
+	}
 
-		if (zio_data_buf_cache[c] != last_data_cache) {
-			last_data_cache = zio_data_buf_cache[c];
-			kmem_cache_destroy(zio_data_buf_cache[c]);
+	for (size_t i = 0; i < n; i++) {
+		kmem_cache_t *cache = zio_data_buf_cache[i];
+		if (cache == NULL)
+			continue;
+		for (size_t j = i; j < n; j++) {
+			if (cache == zio_data_buf_cache[j])
+				zio_data_buf_cache[j] = NULL;
 		}
-		zio_data_buf_cache[c] = NULL;
+		kmem_cache_destroy(cache);
+	}
+
+	for (size_t i = 0; i < n; i++) {
+		VERIFY3P(zio_buf_cache[i], ==, NULL);
+		VERIFY3P(zio_data_buf_cache[i], ==, NULL);
 	}
 
 	kmem_cache_destroy(zio_link_cache);