btrfs-progs: detect discard ability more correctly for btrfs_prepare_device()

Instead of relying on sysfs interface which had some regression
reporting the discard support, let btrfs_prepare_device() to use the
discard_range() result to determine if we need to output the "Performing
full device TRIM" message.

This is done by checking if the first discard succeeded or not.
If the first discard call succeeded, then we know the device support
discard and should output the message.

And to reduce the initial delay before outputting the message (old/lower
end disks may take a long time even discarding 1 GiB), reduce the initial
discard range to 1MiB.

By this it's more reliable to detect discard support for
btrfs_prepare_device() and we can get rid of discard_supported(), and
the timing of the meessage should still be pretty much the same as the
original one, just with a small unobservable delay.

Reviewed-by: Anand Jain <[email protected]>
Reported-by: Anand Jain <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>

Author: adam900710

common/device-utils.c

@@ -60,25 +60,6 @@ static int discard_range(int fd, u64 start, u64 len)
 	return 0;
 }
 
-static int discard_supported(const char *device)
-{
-	int ret;
-	char buf[128] = {};
-
-	ret = device_get_queue_param(device, "discard_granularity", buf, sizeof(buf));
-	if (ret == 0) {
-		pr_verbose(3, "cannot read discard_granularity for %s\n", device);
-		return 0;
-	} else {
-		if (atoi(buf) == 0) {
-			pr_verbose(3, "%s: discard_granularity %s", device, buf);
-			return 0;
-		}
-	}
-
-	return 1;
-}
-
 /*
  * Discard blocks in the given range in 1G chunks, the process is interruptible
  */
@@ -99,6 +80,29 @@ int device_discard_blocks(int fd, u64 start, u64 len)
 	return 0;
 }
 
+static void prepare_discard_device(const char *filename, int fd, u64 byte_count, unsigned opflags)
+{
+	u64 cur = 0;
+
+	while (cur < byte_count) {
+		/* 1G granularity */
+		u64 chunk_size = (cur == 0) ? SZ_1M : min_t(u64, byte_count - cur, SZ_1G);
+		int ret;
+
+		ret = discard_range(fd, cur, chunk_size);
+		if (ret)
+			return;
+		/*
+		 * The first range discarded successfully, meaning the device supports
+		 * discard.
+		 */
+		if (opflags & PREP_DEVICE_VERBOSE && cur == 0)
+			printf("Performing full device TRIM %s (%s) ...\n",
+			       filename, pretty_size(byte_count));
+		cur += chunk_size;
+	}
+}
+
 /*
  * Write zeros to the given range [start, start + len)
  */
@@ -273,17 +277,7 @@ int btrfs_prepare_device(int fd, const char *file, u64 *byte_count_ret,
 			}
 		}
 	} else if (opflags & PREP_DEVICE_DISCARD) {
-		/*
-		 * We intentionally ignore errors from the discard ioctl.  It
-		 * is not necessary for the mkfs functionality but just an
-		 * optimization.
-		 */
-		if (discard_supported(file)) {
-			if (opflags & PREP_DEVICE_VERBOSE)
-				printf("Performing full device TRIM %s (%s) ...\n",
-				       file, pretty_size(byte_count));
-			device_discard_blocks(fd, 0, byte_count);
-		}
+		prepare_discard_device(file, fd, byte_count, opflags);
 	}
 
 	ret = zero_dev_clamped(fd, zinfo, 0, ZERO_DEV_BYTES, byte_count);