linux-ext4 - Re: [PATCH] ext4: introduce linear search for dentries

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Message-ID: <87h64yx4f6.fsf@mailhost.krisman.be>
Date: Wed, 12 Feb 2025 16:02:21 -0500
From: Gabriel Krisman Bertazi <krisman@...e.de>
To: "Theodore Ts'o" <tytso@....edu>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>,  drosen@...gle.com
Subject: Re: [PATCH] ext4: introduce linear search for dentries

"Theodore Ts'o" <tytso@....edu> writes:

> This patch addresses an issue where some files in case-insensitive
> directories become inaccessible due to changes in how the kernel
> function, utf8_casefold(), generates case-folded strings from the
> commit 5c26d2f1d3f5 ("unicode: Don't special case ignorable code
> points").
>
> There are good reasons why this change should be made; it's actually
> quite stupid that Unicode seems to think that the characters ❤ and ❤️
> should be casefolded.  Unfortimately because of the backwards
> compatibility issue, this commit was reverted in 231825b2e1ff.
>
> This problem is addressed by instituting a brute-force linear fallback
> if a lookup fails on case-folded directory, which does result in a
> performance hit when looking up files affected by the changing how
> thekernel treats ignorable Uniode characters, or when attempting to
> look up non-existent file names.  So this fallback can be disabled by
> setting an encoding flag if in the future, the system administrator or
> the manufacturer of a mobile handset or tablet can be sure that there
> was no opportunity for a kernel to insert file names with incompatible
> encodings.
>
> Fixes: 5c26d2f1d3f5 ("unicode: Don't special case ignorable code points")
> Signed-off-by: Theodore Ts'o <tytso@....edu>

Reviewed-by: Gabriel Krisman Bertazi <krisman@...e.de>

-- 
Gabriel Krisman Bertazi