linux-kernel - Re: [PATCH v2] f2fs: Introduce linear search for dentries

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Message-ID: <Z2ketmZiWmUVddpt@infradead.org>
Date: Mon, 23 Dec 2024 00:26:30 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Daniel Lee <chullee@...gle.com>
Cc: Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] f2fs: Introduce linear search for dentries

On Fri, Dec 20, 2024 at 09:21:36AM -0800, Daniel Lee wrote:
> 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").

That also breaks all other direct hashed and needs to be fixed.

> 
> F2FS uses these case-folded names to calculate hash values for locating
> dentries and stores them on disk. Since utf8_casefold() can produce
> different output across kernel versions, stored hash values and newly
> calculated hash values may differ. This results in affected files no
> longer being found via the hash-based lookup.
> 
> To resolve this, the patch introduces a linear search fallback.
> If the initial hash-based search fails, F2FS will sequentially scan the
> directory entries.

That means you have really bad worst case behavior for negative lookups.
I don' think this is in any way a good idea.