linux-ext4 - Re: [PATCH] ext4: add rb_tree cache to struct ext4_group

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Date:	Sun, 4 Apr 2010 14:26:01 +0800
From:	jing zhang <zj.barak@...il.com>
To:	tytso@....edu
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andreas Dilger <adilger@....com>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] ext4: add rb_tree cache to struct ext4_group_info

2010/4/4, tytso@....edu <tytso@....edu>:
> On Sun, Apr 04, 2010 at 09:26:26AM +0800, jing zhang wrote:
>> > That being said, I'm not convinced ext4_mb_generate_from_freelist() is
>> > (a) necessary, or (b) bug-free, either.  The whole point of having
>> > extents in bb_free_root tree is that those extents aren't safe to be
>> > placed in the buddy bitmap.  And ext4_mb_generate_from_freelist()
>> > isn't freeing the nodes from the rbtree.  Fortunately it looks like
>> > ext4_mb_generate_from_freelist is only getting called when the buddy
>> > bitmap is being set up, so the rbtree should be empty during those
>> > times.
>> >
>> > I need to do some more investigation, but I think the function can be
>> > removed entirely.
>>
>> Do you mean that ext4_mb_generate_from_freelist() can be removed entirely?
>
> Maybe.  I need to do more investigation to be sure.  The code in
> mballoc is subtle, and in some places there is stuff which is vestigal
> or misnamed, but it means that I'm going to be very careful before
> changing things.
>
> It also means that if you submit patches, you need to be very clear
> what you think the surrounding code is doing, why you think it's
> wrong, and how your patch make things better.  For example, this:
>
>     The function, ext4_mb_discard_group_preallocations(), works alone as
>     hard as possible without correct understanding its caller's good
>     thinking.
>
>     And now try to relieve it in simple way.
>
> is almost useless as a comment.  It doesn't help me understand the
> code.  "hard as possible"?  Huh?  "without correct understanding"?
> How can code, unless it's artificially intelligent, have
> understanding?  And if you meant the original author had no
> understanding, how do you know that?  "caller's good thinking"?  Same
> question; the calling code doesn't think.
>
> This sort of explanation isn't helpful in understanding the patch.
>
> 							- Ted
>

Another good lesson, I got it.

Thanks
              - zj
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