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	Thu, 09 Nov 2023 19:32:33 +0000
Date: Thu, 9 Nov 2023 19:32:33 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: Christian Brauner <brauner@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 04/22] dentry: switch the lists of children to hlist
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Thu, Nov 09, 2023 at 02:48:19PM +0100, Christian Brauner wrote:
> On Thu, Nov 09, 2023 at 06:20:38AM +0000, Al Viro wrote:
> > Saves a pointer per struct dentry and actually makes the things less
> 
> Which you're giving back to DNAME_INLINE_LEN.

Have to - we want the size to stay a multiple of 64.  So DNAME_INLINE_LEN serves
as a sump - any space savings we get go in there, just as any additional fields
have to pull the space out of there.

FWIW, from distribution of name lengths on 3 local boxen, with reasonably diverse
contents of filesystems:

< 24: 90.6877% 89.8033% 89.3202%
< 25: 92.2120% 90.4324% 90.4652%
< 26: 93.5858% 95.0555% 92.3849%
< 27: 94.6277% 95.4424% 93.1948%
< 28: 95.4827% 95.7796% 93.9134%
< 29: 96.1926% 96.0851% 94.5449%
< 30: 96.7963% 96.3503% 95.1006%
< 32: 97.6930% 96.5792% 95.5681%
< 33: 98.0392% 96.7943% 95.9879%
< 34: 98.3134% 96.9829% 96.3353%
< 35: 98.5493% 97.1352% 96.6313%
< 36: 98.7468% 97.2757% 96.8890%
< 37: 98.9134% 97.4192% 97.1199%
< 38: 99.0515% 97.5506% 97.3372%
< 39: 99.3650% 97.6394% 97.4857%
< 40: 99.4606% 98.8016% 97.7237%

So 32 is tolerable, but going down would rapidly become unpleasant.  This series
does not introduce any new fields, but it's nice to be able to do so without
causing PITA for long names.