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

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Message-ID: <0_14961900_1521456730.19987.cbgrn@bozuman.cybozu.com>
Date:   Mon, 19 Mar 2018 19:52:10 +0900
From:   Shunki Fujita <shunki-fujita@...ozu.co.jp>
To:     Andrew Morton <akpm@...ux-foundation.org>
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

Hi Andrew,

> 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.
> 

This was an unnecessary additional code.
Since bdev-> bd_invalidated always 0 after check_disk_size_change (), no additional code was required.
https://github.com/torvalds/linux/blob/master/fs/block_dev.c#L1365-L1366

I will repost the fix patch.

Thanks,
Shunki