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

[Jaegeuk Kim]

On 12/23, Christoph Hellwig wrote:
> 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.

No surprise. Please check:
https://lore.kernel.org/lkml/Z1mzu4Eg6CPURra3@xxxxxxxxxx/