From: Dave Chinner <david@fromorbit.com>
To: David Sterba <dsterba@suse.cz>
Cc: adilger@dilger.ca, linux-nilfs@vger.kernel.org, mfasheh@suse.com,
xfs@oss.sgi.com, hch@infradead.org, ocfs2-devel@oss.oracle.com,
viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag
Date: Fri, 13 Dec 2013 10:24:43 +1100 [thread overview]
Message-ID: <20131212232443.GL31386@dastard> (raw)
In-Reply-To: <4f8d5dc5b51a43efaf16c39398c23a6276e40a30.1386778303.git.dsterba@suse.cz>
On Thu, Dec 12, 2013 at 04:25:59PM +0100, David Sterba wrote:
> This flag was not accepted when fiemap was proposed [2] due to lack of
> in-kernel users. Btrfs has compression for a long time and we'd like to
> see that an extent is compressed in the output of 'filefrag' utility
> once it's taught about it.
>
> For that purpose, a reserved field from fiemap_extent is used to let the
> filesystem store along the physcial extent length when the flag is set.
> This keeps compatibility with applications that use FIEMAP.
I'd prefer to just see the new physical length field always filled
out, regardless of whether it is a compressed extent or not. In
terms of backwards compatibility to userspace, it makes no
difference because the value of reserved/unused fields is undefined
by the API. Yes, the implementation zeros them, but there's nothing
in the documentation that says "reserved fields must be zero".
Hence I think we should just set it for every extent.
>From the point of view of the kernel API (fiemap_fill_next_extent),
passing the physical extent size in the "len" parameter for normal
extents, then passing 0 for the "physical length" makes absolutely
no sense.
IOWs, what you have created is a distinction between the extent's
"logical length" and it's "physical length". For uncompressed
extents, they are both equal and they should both be passed to
fiemap_fill_next_extent as the same value. Extents where they are
different (i.e. encoded extents) is when they can be different.
Perhaps fiemap_fill_next_extent() should check and warn about
mismatches when they differ and the relevant flags are not set...
> diff --git a/include/uapi/linux/fiemap.h b/include/uapi/linux/fiemap.h
> index 93abfcd..0e32cae 100644
> --- a/include/uapi/linux/fiemap.h
> +++ b/include/uapi/linux/fiemap.h
> @@ -19,7 +19,9 @@ struct fiemap_extent {
> __u64 fe_physical; /* physical offset in bytes for the start
> * of the extent from the beginning of the disk */
> __u64 fe_length; /* length in bytes for this extent */
> - __u64 fe_reserved64[2];
> + __u64 fe_phys_length; /* physical length in bytes, undefined if
> + * DATA_COMPRESSED not set */
> + __u64 fe_reserved64;
> __u32 fe_flags; /* FIEMAP_EXTENT_* flags for this extent */
> __u32 fe_reserved[3];
> };
The comment for fe_length needs to change, too, because it needs to
indicate that it is the logical extent length and that it may be
different to the fe_phys_length depending on the flags that are set
on the extent.
And, FWIW, I wouldn't mention specific flags in the comment here,
but do it at the definition of the flags that indicate there is
a difference between physical and logical extent lengths....
> @@ -50,6 +52,8 @@ struct fiemap {
> * Sets EXTENT_UNKNOWN. */
> #define FIEMAP_EXTENT_ENCODED 0x00000008 /* Data can not be read
> * while fs is unmounted */
> +#define FIEMAP_EXTENT_DATA_COMPRESSED 0x00000040 /* Data is compressed by fs.
> + * Sets EXTENT_ENCODED */
i.e. here.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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next prev parent reply other threads:[~2013-12-12 23:24 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-12 15:25 [PATCH 0/4 v3] fiemap: introduce EXTENT_DATA_COMPRESSED flag David Sterba
2013-12-12 15:25 ` [PATCH 1/4 v3] fiemap: fix comment at EXTENT_DATA_ENCRYPTED David Sterba
2013-12-12 15:25 ` [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag David Sterba
2013-12-12 23:24 ` Dave Chinner [this message]
2013-12-13 0:02 ` Andreas Dilger
2013-12-13 1:22 ` Dave Chinner
2013-12-16 16:49 ` David Sterba
[not found] ` <9520AB36-B728-423A-8EA1-FDD22B79AE90-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>
2014-07-17 6:07 ` Andreas Dilger
[not found] ` <B58DEEA8-561A-4173-B9F5-528B73E06C6D-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>
2014-07-24 19:22 ` David Sterba
2014-07-24 22:34 ` Andreas Dilger
[not found] ` <5B4825C3-F47E-48B7-8DA4-6D79F53B73B1-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>
2014-07-25 6:20 ` Rohan Puri
2014-07-28 16:49 ` [Ocfs2-devel] " David Sterba
2013-12-13 11:06 ` Christoph Hellwig
2013-12-12 15:26 ` [PATCH 3/4 v3] btrfs: set FIEMAP_EXTENT_DATA_COMPRESSED for compressed extents David Sterba
2013-12-12 22:20 ` Andreas Dilger
2013-12-12 15:26 ` [PATCH 4/4 v3] Documentation/fiemap: Document the DATA_COMPRESSED flag David Sterba
2013-12-12 22:22 ` [PATCH 0/4 v3] fiemap: introduce EXTENT_DATA_COMPRESSED flag Andreas Dilger
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