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From: Greg Freemyer <greg.freemyer@gmail.com>
Date: Tue, 16 Nov 2010 12:47:20 -0800
Message-ID: <AANLkTikz07C_=aZudup7xSpLnrEYi5No4QhL5WD9xxTx@mail.gmail.com>
Subject: Re: [PATCH 4/6] Ext4: fail if we try to use hole punch
To: Josef Bacik <josef@redhat.com>
Cc: Avi Kivity <avi@redhat.com>, david@fromorbit.com,
        linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        xfs@oss.sgi.com, cmm@us.ibm.com, cluster-devel@redhat.com,
        ocfs2-devel@oss.oracle.com
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On Tue, Nov 16, 2010 at 8:05 AM, Josef Bacik <josef@redhat.com> wrote:
> On Tue, Nov 16, 2010 at 03:07:29PM +0200, Avi Kivity wrote:
>> On 11/16/2010 02:50 PM, Josef Bacik wrote:
>>> On Tue, Nov 16, 2010 at 02:25:35PM +0200, Avi Kivity wrote:
>>> > ?On 11/15/2010 07:05 PM, Josef Bacik wrote:
>>> >> ?Ext4 doesn't have the ability to punch holes yet, so make sure we return
>>> >> ?EOPNOTSUPP if we try to use hole punching through fallocate. ?This support can
>>> >> ?be added later. ?Thanks,
>>> >>
>>> >
>>> > ?Instead of teaching filesystems to fail if they don't support the
>>> > ?capability, why don't supporting filesystems say so, allowing the fail
>>> > ?code to be in common code?
>>> >
>>>
>>> There is no simple way to test if a filesystem supports hole punching or not so
>>> the check has to be done per fs. ?Thanks,
>>
>> Could put a flag word in superblock_operations. ?Filesystems which
>> support punching (or other features) can enable it there.
>>
>> Or even have its own callback.
>>
>
> Sure but then you have to do the same thing for every other flag you add to
> fallocate and then you have this huge mess of random flags just so you don't
> call into the filesystem. ?This way is a lesser of two evils I think. ?Thanks,
>
> Josef

I'm not a true kernel hacker, so my opinion is not critical but I find
it hard to read / expand as

> +     /* We only support the FALLOC_FL_KEEP_SIZE mode */
> +     if (mode && (mode != FALLOC_FL_KEEP_SIZE))
> +             return -EOPNOTSUPP;

How about:

#define EXT4_FALLOC_MODES_SUPPORTED (FALLOC_FL_KEEP_SIZE)

if (modes & ~EXT4_FALLOC_MODES_SUPPORTED)
             return -EOPNOTSUPP;


Greg
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