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

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Message-ID: <20190811161508.GA5878@mit.edu>
Date:   Sun, 11 Aug 2019 12:15:08 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 Sat, Aug 10, 2019 at 12:33:43AM -0700, Christoph Hellwig wrote:
> 
> On Fri, Aug 09, 2019 at 02:18:29PM -0400, Theodore Ts'o wrote:
> > For debugging reasons, it's useful to know the contents of the extent
> > cache.  Since the extent cache contains much of what is in the fiemap
> > ioctl, extend the fiemap interface to return this information via some
> > ext4-specific flags.
> 
> 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'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".

						- Ted