Re: [PATCH] Add support to check for FALLOC_FL_COLLAPSE_RANGE and FALLOC_FL_ZERO_RANGE crap modes

[Nick Krause]

On Fri, Aug 1, 2014 at 8:21 AM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> On Thu, Jul 31, 2014 at 08:09:10PM +0100, Hugo Mills wrote:
>> On Thu, Jul 31, 2014 at 01:53:33PM -0400, Nicholas Krause wrote:
>> > This adds checks for the stated modes as if they are crap we will return error
>> > not supported.
>>
>>    You've just enabled two options, but you haven't actually
>> implemented the code behind it. I would tell you *NOT* to do anything
>> else on this work until you can answer the question: What happens if
>> you apply this patch, create a large file called "foo.txt", and then a
>> userspace program executes the following code?
>>
>> int fd = open("foo.txt", O_RDWR);
>> fallocate(fd, FALLOCATE_FL_COLLAPSE_RANGE, 50, 50);
>>
>>    Try it on a btrfs filesystem, both with and without your patch.
>> Also try it on an ext4 filesystem.
>>
>>    Once you've done all of that, reply to this mail and tell me what
>> the problem is with this patch. You need to make two answers: what are
>> the technical problems with the patch? What errors have you made in
>> the development process?
>
> There are also the conceptual failures.  Before you do anything else,
> you need to be able to answer the question, "what do you think the
> flags FALLOC_FL_COLLAPSE_RANGE and FALLOC_FL_ZERO_RANGE are supposed
> to do?"  What are the possible appropriate things for btrfs to do if
> it sees these flags?  (Hint: there is more than one correct answer,
> and its current choice is one of them.  What is the other one?)
>
> Nick, the fact that you call these modes "crap" is a hint that you
> have a fundamental lack of understanding --- and before you waste more
> of kernel developers' time, you need to get that understanding first,
> for any bit of code that you propose to "improve".
>
> This is why I suggested that you work on userspace testing scripts
> first.  It's pretty clear you are (a) incredibly sloppy, and (b)
> lacking conceptual understanding of a lot of technical details, and
> (c) even worse, aren't letting this lack of understanding stop you
> from posting patches.  As a result you are adding negative value to
> whatever project or subsystem you try to attach yourself to --- you're
> not helping.
>
>                                                 - Ted
>
> P.S.   As a further hint, change the above code to read:
>
>         int fd = open("foo.txt", O_RDWR);
>         if (fallocate(fd, FALLOCATE_FL_COLLAPSE_RANGE, 4096, 8192) < 0)
>                 perror("fallocate");
>
> And then run "filefrag -vs foo.txt" before and after running the above
> code fragment and then try something like this:
>
>      cp /usr/share/dict/words foo.txt
>      filefrag -vs foo.txt
>      ls -l foo.txt
>      /tmp/fallocate-test-prog
>      filefrag -vs foo.txt
>      ls -l foo.txt
>      diff /usr/share/dict/words foo.txt
>
> Try doing this on an ext4 or xfs system and a btrfs file system.

I miss send this patch, that's my there are issues.
Cheers Nick
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