Re: md related oops triggered in bdev_inode_switch_bdi

[Wu Fengguang]

Hi Neil,

> Subject: [PATCH] Avoid dereferencing a 'request_queue' after last close.

Reviewed-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>

with comments below.

> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1430,6 +1430,12 @@ static int __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
>  		sync_blockdev(bdev);
>  		kill_bdev(bdev);
>  	}
> +	if (!bdev->bd_openers)
> +		/* ->release can cause the old bdi to disappear,
> +		 * so must switch it out first
> +		 */
> +		bdev_inode_switch_bdi(bdev->bd_inode,
> +					&default_backing_dev_info);
>  	if (bdev->bd_contains == bdev) {
>  		if (disk->fops->release)
>  			ret = disk->fops->release(disk, mode);

The bdev_inode_switch_bdi() call can be further moved into the
previous if block, like this:

        if (!--bdev->bd_openers) {
                WARN_ON_ONCE(bdev->bd_holders);
                sync_blockdev(bdev);
                kill_bdev(bdev);
+
+               /* ->release can cause the old bdi to disappear,
+                * so must switch it out first
+                */
+               bdev_inode_switch_bdi(bdev->bd_inode,
+                                       &default_backing_dev_info);
        }

Then it's obvious that kill_bdev() will truncate all inode pages
and there won't be further interactions with dirty writes.

Although there are dozens of disk->fops->release functions, however
it's very unlikely they need to access some inode on top of the disk
(which is illogical thing).

So I don't see any problems. It makes sense to push it to next for
broader test ASAP. Will you do it, or me?

Thanks,
Fengguang
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