linux-kernel - Re: [PATCH 1/4] exfat: Simplify exfat_utf8_d

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Message-ID: <20200328234057.dh23w255eaag2glg@pali>
Date:   Sun, 29 Mar 2020 00:40:57 +0100
From:   Pali Rohár <pali@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Namjae Jeon <namjae.jeon@...sung.com>,
        Sungjong Seo <sj1557.seo@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] exfat: Simplify exfat_utf8_d_hash() for code points
 above U+FFFF

On Wednesday 18 March 2020 10:32:51 Pali Rohár wrote:
> On Wednesday 18 March 2020 00:09:25 Al Viro wrote:
> > On Tue, Mar 17, 2020 at 11:25:52PM +0100, Pali Rohár wrote:
> > > Function partial_name_hash() takes long type value into which can be stored
> > > one Unicode code point. Therefore conversion from UTF-32 to UTF-16 is not
> > > needed.
> > 
> > Hmm...  You might want to update the comment in stringhash.h...
> 
> Well, initially I have not looked at hashing functions deeply. Used
> hashing function in stringhash.h is defined as:
> 
> static inline unsigned long
> partial_name_hash(unsigned long c, unsigned long prevhash)
> {
> 	return (prevhash + (c << 4) + (c >> 4)) * 11;
> }
> 
> I guess it was designed for 8bit types, not for long (64bit types) and
> I'm not sure how effective it is even for 16bit types for which it is
> already used.
> 
> So question is, what should we do for either 21bit number (one Unicode
> code point = equivalent of UTF-32) or for sequence of 16bit numbers
> (UTF-16)?
> 
> Any opinion?

So what to do with that hashing function?

Anyway, "[PATCH 4/4] exfat: Fix discard support" should be reviewed as
currently discard support in exfat is broken.