Date:	Fri, 25 Jul 2014 11:50:58 +0530
From:	Rohan Puri <rohan.puri15@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	dsterba@...e.cz, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-nilfs@...r.kernel.org, xfs@....sgi.com,
	Btrfs Developer List <linux-btrfs@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	ocfs2-devel@....oracle.com
Subject: Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag

On Fri, Jul 25, 2014 at 4:04 AM, Andreas Dilger <adilger@...ger.ca> wrote:
>
> On Jul 24, 2014, at 1:22 PM, David Sterba <dsterba@...e.cz> wrote:
>> On Thu, Jul 17, 2014 at 12:07:57AM -0600, Andreas Dilger wrote:
>>> any progress on this patch series?
>>
>> I'm sorry I got distracted at the end of year and did not finish the
>> series.
>>
>>> I never saw an updated version of this patch series after the last round of
>>> reviews, but it would be great to move it forward.  I have filefrag patches
>>> in my e2fsprogs tree waiting for an updated version of your patch.
>>>
>>> I recall the main changes were:
>>> - add FIEMAP_EXTENT_PHYS_LENGTH flag to indicate if fe_phys_length was valid
>>
>> fe_phys_length will be always valid, so other the flags are set only if it's
>> not equal to the logical length.
>>
>>> - rename fe_length to fe_logi_length and #define fe_length fe_logi_length
>>> - always fill in fe_phys_length (= fe_logi_length for uncompressed files)
>>>  and set FIEMAP_EXTENT_PHYS_LENGTH whether the extent is compressed or not
>>
>> This is my understanding and contradicts the first point.
>
> I think Dave Chinner's former point was that having fe_phys_length validity
> depend on FIEMAP_EXTENT_DATA_COMPRESSED is a non-intuitive interface.  It is
> not true that fe_phys_length would always be valid, since that is not the
> case for older kernels that currently always set this field to 0, so they
> need some flag to indicate if fe_phys_length is valid.  Alternately,
> userspace could do:
>
>         if (ext->fe_phys_length == 0)
>                 ext->fe_phys_length = ext->fe_logi_length;
>
> but that pre-supposes that fe_phys_length == 0 is never a valid value when
> fe_logi_length is non-zero, and this might introduce errors in some cases.
> I could imagine that some compression methods might not allocate any space
> at all if it was all zeroes, and just store a bit in the blockpointer or
> extent, so having a separate FIEMAP_EXTENT_PHYS_LENGTH is probably safer
> in the long run.
zfs is an example of this.
> That opens up the question of whether a written zero
> filled space that gets compressed away is different from a hole, but I'd
> prefer to just return whatever the file mapping is than interpret it.
>
> Cheers, Andreas
>
>>> - add WARN_ONCE() in fiemap_fill_next_extent() as described below
>>>
>>> I don't know if there was any clear statement about whether there should be
>>> separate FIEMAP_EXTENT_PHYS_LENGTH and FIEMAP_EXTENT_DATA_COMPRESSED flags,
>>> or if the latter should be implicit?  Probably makes sense to have separate
>>> flags.  It should be fine to use:
>>>
>>> #define FIEMAP_EXTENT_PHYS_LENGTH    0x00000010
>>>
>>> since this flag was never used.
>>
>> I've kept only FIEMAP_EXTENT_DATA_COMPRESSED, I don't see a need for
>> FIEMAP_EXTENT_PHYS_LENGTH and this would be yet another flag because the
>> FIEMAP_EXTENT_DATA_ENCODED is also implied.
>>
>> I'll send V4, we can discuss the PHYS_LENGTH flag then.
>
>
> Cheers, Andreas
>
>
>
>
>

Regards,
Rohan
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