linux-ext4 - Re: [PATCH 1/3] ext4: return the extent cache information via fiemap

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Date:   Sun, 11 Aug 2019 23:47:28 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH 1/3] ext4: return the extent cache information via fiemap

On Sun, Aug 11, 2019 at 12:15:08PM -0400, Theodore Y. Ts'o wrote:
> > Nak.  No weird fs specific fiemap flags that aren't even in the uapi
> > header.  Please provide your own debug only interface.
> 
> I can understand why you don't like this from the principle of the
> thing.

I think mixing up in-kernel temporary state with the on-disk layout
is a bad idea, yes.  I think it is an even worse idea to try to sneak
it in without API review in an undocumented fashion.

> 
> I'll create my own ioctl, and make a copy of ioctl_fiemap() into ext4
> and modify it for my needs.  I was trying to avoid needing to do that,
> since there is plenty of space in the fiemap flags to carve out space
> for file-specific specific flags, and avoiding making extra copies of
> code for the purposes of reuse weighed more heavily than "no
> fs-specific fiemap flags".

I think dumping your internal state is a candidate for debugs, not
a copy of ioctl_fiemap.