linux-kernel - Re: [PATCH v7] iomap: make inline data support more flexible

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Message-ID: <YQALsvt0UWGW+iMw@casper.infradead.org>
Date:   Tue, 27 Jul 2021 14:35:46 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     dsterba@...e.cz, Christoph Hellwig <hch@....de>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Gao Xiang <hsiangkao@...ux.alibaba.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Huang Jianan <huangjianan@...o.com>,
        linux-erofs@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andreas Gruenbacher <andreas.gruenbacher@...il.com>
Subject: Re: [PATCH v7] iomap: make inline data support more flexible

On Tue, Jul 27, 2021 at 10:20:42AM +0200, David Sterba wrote:
> On Mon, Jul 26, 2021 at 02:17:02PM +0200, Christoph Hellwig wrote:
> > > Subject: iomap: Support tail packing
> > 
> > I can't say I like this "tail packing" language here when we have the
> > perfectly fine inline wording.  Same for various comments in the actual
> > code.
> 
> Yes please, don't call it tail-packing when it's an inline extent, we'll
> use that for btrfs eventually and conflating the two terms has been
> cofusing users. Except reiserfs, no linux filesystem does tail-packing.

Hmm ... I see what reiserfs does as packing tails of multiple files into
one block.  What gfs2 (and ext4) do is inline data.  Erofs packs the
tail of a single file into the same block as the inode.  If I understand
what btrfs does correctly, it stores data in the btree.  But (like
gfs2/ext4), it's only for the entire-file-is-small case, not for
its-just-ten-bytes-into-the-last-block case.

So what would you call what erofs is doing if not tail-packing?
Wikipedia calls it https://en.wikipedia.org/wiki/Block_suballocation
which doesn't quite fit.  We need a phrase which means "this isn't
just for small files but for small tails of large files".