linux-kernel - Re: [PATCH] fs: don't flush pagecache when expanding block device

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Date:   Fri, 16 Mar 2018 15:22:56 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     shunki-fujita <shunki-fujita@...ozu.co.jp>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fs: don't flush pagecache when expanding block device

On Fri, 16 Mar 2018 15:55:53 +0900 shunki-fujita <shunki-fujita@...ozu.co.jp> wrote:

> When changing the size of a block device, its all caches are freed.
> It's necessary on shrinking to prevent spurious I/Os to the disappeared region.
> However, on expanding, such kind of I/Os doesn't happen.
> 
> Similar things can be considered for btrfs filesystem resize and resize2fs,
> but they are designed not to cache drops when expanding.
> Therefore this patch removes unnecessary cache drop.

Yes, the patch removes the flush_disk() call when shrinking.  But it
adds additional code which is unchangelogged and uncommented.  What's
happening here?

> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1337,7 +1337,14 @@ void check_disk_size_change(struct gendisk *disk, struct block_device *bdev)
>  		       "%s: detected capacity change from %lld to %lld\n",
>  		       disk->disk_name, bdev_size, disk_size);
>  		i_size_write(bdev->bd_inode, disk_size);
> -		flush_disk(bdev, false);
> +		if (bdev_size > disk_size) {
> +			flush_disk(bdev, false);
> +		} else {
> +			if (!bdev->bd_disk)
> +				return;
> +			if (disk_part_scan_enabled(bdev->bd_disk))
> +				bdev->bd_invalidated = 1;
> +		}
>  	}
>  }

Oh, I see.  It's a copy-n-paste of some mystery uncommented,
lost-in-the-mists-of-time code from flush_disk().  Argh.

The world would be a better place if we could move that bit of code
into a separate function, figure out what it does, document it and call
it from both sites.