loop: avoid EAGAIN, if offset or block_size are changed

Previously, there was a bug where user could see stale buffer cache (e.g, 512B)
attached in the 4KB-sized pager cache, when the block size was changed from
512B to 4KB. That was fixed by:
commit 5db470e ("loop: drop caches if offset or block_size are changed")

But, there were some regression reports saying the fix returns EAGAIN easily.
So, this patch removes previously added EAGAIN condition, nrpages != 0.

Instead, it changes the flow like this:
- sync_blockdev()
- blk_mq_freeze_queue()
 : change the loop configuration
- blk_mq_unfreeze_queue()
- sync_blockdev()
- invalidate_bdev()

After invalidating the buffer cache, we must see the full valid 4KB page.

Additional concern came from Bart in which we can lose some data when
changing the lo_offset. In that case, this patch adds:
- sync_blockdev()
- blk_set_queue_dying
- blk_mq_freeze_queue()
 : change the loop configuration
- blk_mq_unfreeze_queue()
- blk_queue_flag_clear(QUEUE_FLAG_DYING);
- sync_blockdev()
- invalidate_bdev()

Report: https://bugs.chromium.org/p/chromium/issues/detail?id=938958#c38

Cc: <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: [email protected]
Cc: Bart Van Assche <[email protected]>
Fixes: 5db470e ("loop: drop caches if offset or block_size are changed")
Reported-by: Gwendal Grignou <[email protected]>
Reported-by: grygorii tertychnyi <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

drivers/block/loop.c

@@ -1247,6 +1247,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	kuid_t uid = current_uid();
 	struct block_device *bdev;
 	bool partscan = false;
+	bool drop_request = false;
+	bool drop_cache = false;
 
 	err = mutex_lock_killable(&loop_ctl_mutex);
 	if (err)
@@ -1266,14 +1268,21 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 		goto out_unlock;
 	}
 
+	if (lo->lo_offset != info->lo_offset)
+		drop_request = true;
 	if (lo->lo_offset != info->lo_offset ||
-	    lo->lo_sizelimit != info->lo_sizelimit) {
-		sync_blockdev(lo->lo_device);
-		kill_bdev(lo->lo_device);
-	}
+	    lo->lo_sizelimit != info->lo_sizelimit)
+		drop_cache = true;
 
-	/* I/O need to be drained during transfer transition */
-	blk_mq_freeze_queue(lo->lo_queue);
+	sync_blockdev(lo->lo_device);
+
+	if (drop_request) {
+		blk_set_queue_dying(lo->lo_queue);
+		blk_mq_freeze_queue_wait(lo->lo_queue);
+	} else {
+		/* I/O need to be drained during transfer transition */
+		blk_mq_freeze_queue(lo->lo_queue);
+	}
 
 	err = loop_release_xfer(lo);
 	if (err)
@@ -1300,14 +1309,6 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 
 	if (lo->lo_offset != info->lo_offset ||
 	    lo->lo_sizelimit != info->lo_sizelimit) {
-		/* kill_bdev should have truncated all the pages */
-		if (lo->lo_device->bd_inode->i_mapping->nrpages) {
-			err = -EAGAIN;
-			pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
-				__func__, lo->lo_number, lo->lo_file_name,
-				lo->lo_device->bd_inode->i_mapping->nrpages);
-			goto out_unfreeze;
-		}
 		if (figure_loop_size(lo, info->lo_offset, info->lo_sizelimit)) {
 			err = -EFBIG;
 			goto out_unfreeze;
@@ -1344,6 +1345,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 
 out_unfreeze:
 	blk_mq_unfreeze_queue(lo->lo_queue);
+	if (drop_request)
+		blk_queue_flag_clear(QUEUE_FLAG_DYING, lo->lo_queue);
 
 	if (!err && (info->lo_flags & LO_FLAGS_PARTSCAN) &&
 	     !(lo->lo_flags & LO_FLAGS_PARTSCAN)) {
@@ -1352,6 +1355,12 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 		bdev = lo->lo_device;
 		partscan = true;
 	}
+
+	/* truncate stale pages cached by previous operations */
+	if (!err && drop_cache) {
+		sync_blockdev(lo->lo_device);
+		invalidate_bdev(lo->lo_device);
+	}
 out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
 	if (partscan)
@@ -1533,8 +1542,6 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg)
 
 static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
 {
-	int err = 0;
-
 	if (lo->lo_state != Lo_bound)
 		return -ENXIO;
 
@@ -1545,27 +1552,18 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
 		return 0;
 
 	sync_blockdev(lo->lo_device);
-	kill_bdev(lo->lo_device);
-
 	blk_mq_freeze_queue(lo->lo_queue);
 
-	/* kill_bdev should have truncated all the pages */
-	if (lo->lo_device->bd_inode->i_mapping->nrpages) {
-		err = -EAGAIN;
-		pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
-			__func__, lo->lo_number, lo->lo_file_name,
-			lo->lo_device->bd_inode->i_mapping->nrpages);
-		goto out_unfreeze;
-	}
-
 	blk_queue_logical_block_size(lo->lo_queue, arg);
 	blk_queue_physical_block_size(lo->lo_queue, arg);
 	blk_queue_io_min(lo->lo_queue, arg);
 	loop_update_dio(lo);
-out_unfreeze:
 	blk_mq_unfreeze_queue(lo->lo_queue);
 
-	return err;
+	/* truncate stale pages cached by previous operations */
+	sync_blockdev(lo->lo_device);
+	invalidate_bdev(lo->lo_device);
+	return 0;
 }
 
 static int lo_simple_ioctl(struct loop_device *lo, unsigned int cmd,