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@xxxxxxxxxxxxx
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