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Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag

To: Andreas Dilger <adilger@xxxxxxxxx>
Subject: Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag
From: Rohan Puri <rohan.puri15@xxxxxxxxx>
Date: Fri, 25 Jul 2014 11:50:58 +0530
Cc: dsterba@xxxxxxx, linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>, linux-nilfs@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, Btrfs Developer List <linux-btrfs@xxxxxxxxxxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, ocfs2-devel@xxxxxxxxxxxxxx
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References: <cover.1386778302.git.dsterba@xxxxxxx> <4f8d5dc5b51a43efaf16c39398c23a6276e40a30.1386778303.git.dsterba@xxxxxxx> <20131212232443.GL31386@dastard> <9520AB36-B728-423A-8EA1-FDD22B79AE90@xxxxxxxxx> <B58DEEA8-561A-4173-B9F5-528B73E06C6D@xxxxxxxxx> <20140724192245.GG1553@xxxxxxx> <5B4825C3-F47E-48B7-8DA4-6D79F53B73B1@xxxxxxxxx>
On Fri, Jul 25, 2014 at 4:04 AM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
> On Jul 24, 2014, at 1:22 PM, David Sterba <dsterba@xxxxxxx> 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
>>> 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


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