Date:	Sun, 5 Nov 2006 05:14:06 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: New filesystem for Linux

On Sat, 4 Nov 2006, Linus Torvalds wrote:

> On Thu, 2 Nov 2006, Mikulas Patocka wrote:
>>
>> As my PhD thesis, I am designing and writing a filesystem, and it's now in a
>> state that it can be released. You can download it from
>> http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/
>
> Ok, not having actually tested any of this, I only have a few comments on
> the source code:
>
> - the source tree layout is very confusing. Can you please separate the
>   mkfs/fsck parts more clearly from the kernel driver parts?

Yes, fsck is already separated, mkfs could be too.

> - you have a _very_ confusing usage of upper-case. Not only are a lot of
>   functions upper-case, some filenames are also upper-case. What would
>   otherwise be more readable just ends up being hard to read because it's
>   so odd and unexpected.
>
>   I'm sure there is some logic to it, but it escapes me.

I'm used to this. I usually make important functions with uppercase 
letters and nonimportant temporary functions with lowercase letters.

But I see that it contradicts general kernel coding style, so I can change 
it.

BTW do you find uppercase typedefs like
typedef struct {
 	...
} SPADFNODE;
confusing too?

Uppercase filenames are there because the files are taken from another 
(not yet released) project. But the kernel driver does not share any code 
except definitions of disk structures, I saw how badly an attempt to share 
kernel code affected XFS.

> - your whitespace usage needs some work: please put empty lines between
>   the declarations and the code in a function, and since you use a fair
>   amount of "goto"s, please do NOT indent them into the code (it's almost
>   impossible to pick out the target labels because you hide them with the
>   code).
>
> - your whitespace, part 2: you have a fair number of one-liner
>   if-statements, where again there is no indentation, and thus the flow
>   is almost impossible to see. Don't wrote
>
> 	if (somecomplexconditional) return;
>
>   but please instead write
>
> 	if (somecomplexcondifional)
> 		return;
>
>   and perhaps use a few more empty lines to separate out the "paragraphs"
>   of code (the same way you write email - nobody wants to see one solid
>   block of code, you'd prefer to see "logical sections").
>
>   Here's a prime example of what NOT to do:
>
> 	if (__likely(!(((*c)[1] - 1) & (*c)[1]))) (*c)[0] = key;
>
>   I dare anybody to be able to read that. That wasn't even the worst one:
>   some of those if-statements were so long that you couldn't even _see_
>   what the statement inside the if-statement even was (and I don't use a
>   80-column wide terminal, this was in a 112-column xterm)

I see, that is fixable easily.

> - why use "__d_off" etc hard-to-read types? You seem to have typedef'ed
>   it from sector_t, but you use a harder-to-read name than the original
>   type was. Hmm?

I am used to __d_off from elsewhere. The same reason why I use 
__likely/__unlikely instead of likely/unlikely.

__d_off may have some little meaning --- if someone wants to run 32-bit 
spadfs filesystem on a kernel configuration with 64-bit sector_t. But I'm 
not sure if someone would ever want it.

> - you have a few comments, but you could have a lot more explanation,
>   especially since not all of your names are all that self-explanatory.
>
> Ok, with that out of the way, let's say what I _like_ about it:
>
> - it's fairly small
>
> - the code, while having the above problems, looks generally fairly
>   clean. The whitespace issues get partially cleared by just running
>   "Lindent" on it, although that's not perfect either (it still indents
>   the goto target labels too much, although it at least makes them
>   _visible_. But it won't add empty lines to delineate sections, of
>   course, and it doesn't add comments ;^)
>
> - I like a lot of the notions, and damn, small and simple are both
>   virtues on their own.
>
> So if you could make the code easier to read, and were to do some
> benchmarking to show what it's good at and what the problems are, I think
> you'd find people looking at it. It doesn't look horrible to me.

I placed some benchmark on 
http://artax.karlin.mff.cuni.cz/~mikulas/spadfs/benchmarks/

The main shortcoming: slow fsync. fsync on spadfs generally has to flush 
all metadata buffers (it could be improved at least for case when file 
size does not change --- for databases).

Mikulas

> 		Linus
>
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