linux-kernel - Re: [PATCH v7 1/8] unicode: Add utf8_casefold

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Date:   Mon, 17 Feb 2020 14:02:10 -0500
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Daniel Rosenberg <drosen@...gle.com>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        "Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fscrypt@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 1/8] unicode: Add utf8_casefold_iter

Daniel Rosenberg <drosen@...gle.com> writes:

> On Tue, Feb 11, 2020 at 7:38 PM Eric Biggers <ebiggers@...nel.org> wrote:
>>
>> Indirect function calls are expensive these days for various reasons, including
>> Spectre mitigations and CFI.  Are you sure it's okay from a performance
>> perspective to make an indirect call for every byte of the pathname?
>>
>> > +typedef int (*utf8_itr_actor_t)(struct utf8_itr_context *, int byte, int pos);
>>
>> The byte argument probably should be 'u8', to avoid confusion about whether it's
>> a byte or a Unicode codepoint.
>>

just for the record, we use int utf8byte because it can fail
error codes, but that is not the case here.  It should be u8.

>
> Gabriel, what do you think here? I could change it to either exposing
> the things necessary to do the hashing in libfs, or instead of the
> general purpose iterator, just have a hash function inside of unicode
> that will compute the hash given a seed value.

Sorry for the delay, I'm away on a long vacation and intentionally
staying away from my laptop :)

Eric has a very good point, if not prohibitively, it is unnecessarily
expensive for a hot path.  Why not expose utf8ncursor and utf8byte to
libfs and implement the hash in libfs?

-- 
Gabriel Krisman Bertazi