linux-kernel - Re: [PATCH] Add support to check for FALLOC_FL_COLLAPSE_RANGE and FALLOC_FL_ZERO

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Message-ID: <20140731190910.GP31950@carfax.org.uk>
Date:	Thu, 31 Jul 2014 20:09:10 +0100
From:	Hugo Mills <hugo@...fax.org.uk>
To:	Nicholas Krause <xerofoify@...il.com>
Cc:	clm@...com, jbacik@...com, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add support to check for FALLOC_FL_COLLAPSE_RANGE  and
 FALLOC_FL_ZERO_RANGE crap modes

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?

   *Only* if you can answer those questions sensibly, should you write
any more patches, of any kind.

   Hugo.

> Signed-off-by: Nicholas Krause <xerofoify@...il.com>
> ---
>  fs/btrfs/file.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 1f2b99c..599495a 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -2490,7 +2490,8 @@ static long btrfs_fallocate(struct file *file, int mode,
>  	alloc_end = round_up(offset + len, blocksize);
>  
>  	/* Make sure we aren't being give some crap mode */
> -	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
> +	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE|
> +	FALLOC_FL_COLLAPSE_RANGE | FALLOC_FL_ZERO_RANGE))
>  		return -EOPNOTSUPP;
>  
>  	if (mode & FALLOC_FL_PUNCH_HOLE)
> -- 
> 1.7.10.4
> 
> --
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-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
  --- The glass is neither half-full nor half-empty; it is twice as ---  
                        large as it needs to be.                         

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