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

To: David Sterba <dsterba@xxxxxxx>
Subject: Re: [PATCH 2/4 v3] fiemap: add EXTENT_DATA_COMPRESSED flag
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 13 Dec 2013 10:24:43 +1100
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, adilger@xxxxxxxxx, linux-nilfs@xxxxxxxxxxxxxxx, mfasheh@xxxxxxxx, xfs@xxxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx
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In-reply-to: <4f8d5dc5b51a43efaf16c39398c23a6276e40a30.1386778303.git.dsterba@xxxxxxx>
References: <cover.1386778302.git.dsterba@xxxxxxx> <4f8d5dc5b51a43efaf16c39398c23a6276e40a30.1386778303.git.dsterba@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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.


Dave Chinner

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