Re: [PATCH v3 01/13] fs: fiemap: add physical_length field to extents

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Apr 03, 2024 at 03:22:42AM -0400, Sweet Tea Dorminy wrote:
> Some filesystems support compressed extents which have a larger logical
> size than physical, and for those filesystems, it can be useful for
> userspace to know how much space those extents actually use. For
> instance, the compsize [1] tool for btrfs currently uses btrfs-internal,
> root-only ioctl to find the actual disk space used by a file; it would
> be better and more useful for this information to require fewer
> privileges and to be usable on more filesystems. Therefore, use one of
> the padding u64s in the fiemap extent structure to return the actual
> physical length; and, for now, return this as equal to the logical
> length.
> 
> [1] https://github.com/kilobyte/compsize
> 
> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
> ---
>  Documentation/filesystems/fiemap.rst | 28 +++++++++++++++++-------
>  fs/ioctl.c                           |  3 ++-
>  include/uapi/linux/fiemap.h          | 32 ++++++++++++++++++++++------
>  3 files changed, 47 insertions(+), 16 deletions(-)
> 
> diff --git a/Documentation/filesystems/fiemap.rst b/Documentation/filesystems/fiemap.rst
> index 93fc96f760aa..c2bfa107c8d7 100644
> --- a/Documentation/filesystems/fiemap.rst
> +++ b/Documentation/filesystems/fiemap.rst
> @@ -80,14 +80,24 @@ Each extent is described by a single fiemap_extent structure as
>  returned in fm_extents::
>  
>      struct fiemap_extent {
> -	    __u64	fe_logical;  /* logical offset in bytes for the start of
> -				* the extent */
> -	    __u64	fe_physical; /* physical offset in bytes for the start
> -				* of the extent */
> -	    __u64	fe_length;   /* length in bytes for the extent */
> -	    __u64	fe_reserved64[2];
> -	    __u32	fe_flags;    /* FIEMAP_EXTENT_* flags for this extent */
> -	    __u32	fe_reserved[3];
> +            /*
> +             * logical offset in bytes for the start of
> +             * the extent from the beginning of the file
> +             */
> +            __u64 fe_logical;
> +            /*
> +             * physical offset in bytes for the start
> +             * of the extent from the beginning of the disk
> +             */
> +            __u64 fe_physical;
> +            /* logical length in bytes for this extent */
> +            __u64 fe_logical_length;
> +            /* physical length in bytes for this extent */
> +            __u64 fe_physical_length;
> +            __u64 fe_reserved64[1];
> +            /* FIEMAP_EXTENT_* flags for this extent */
> +            __u32 fe_flags;
> +            __u32 fe_reserved[3];
>      };
>  
>  All offsets and lengths are in bytes and mirror those on disk.  It is valid
> @@ -175,6 +185,8 @@ FIEMAP_EXTENT_MERGED
>    userspace would be highly inefficient, the kernel will try to merge most
>    adjacent blocks into 'extents'.
>  
> +FIEMAP_EXTENT_HAS_PHYS_LEN
> +  This will be set if the file system populated the physical length field.

Just out of curiosity, should filesystems set this flag and
fe_physical_length if fe_physical_length == fe_logical_length?
Or just leave both blank?

>  VFS -> File System Implementation
>  ---------------------------------
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 661b46125669..8afd32e1a27a 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -138,7 +138,8 @@ int fiemap_fill_next_extent(struct fiemap_extent_info *fieinfo, u64 logical,
>  	memset(&extent, 0, sizeof(extent));
>  	extent.fe_logical = logical;
>  	extent.fe_physical = phys;
> -	extent.fe_length = len;
> +	extent.fe_logical_length = len;
> +	extent.fe_physical_length = len;
>  	extent.fe_flags = flags;
>  
>  	dest += fieinfo->fi_extents_mapped;
> diff --git a/include/uapi/linux/fiemap.h b/include/uapi/linux/fiemap.h
> index 24ca0c00cae3..3079159b8e94 100644
> --- a/include/uapi/linux/fiemap.h
> +++ b/include/uapi/linux/fiemap.h
> @@ -14,14 +14,30 @@
>  
>  #include <linux/types.h>
>  
> +/*
> + * For backward compatibility, where the member of the struct was called
> + * fe_length instead of fe_logical_length.
> + */
> +#define fe_length fe_logical_length

This #define has global scope; are you sure this isn't going to cause a
weird build problem downstream with some program that declares an
unrelated fe_length symbol?

> +
>  struct fiemap_extent {
> -	__u64 fe_logical;  /* logical offset in bytes for the start of
> -			    * the extent from the beginning of the file */
> -	__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];
> -	__u32 fe_flags;    /* FIEMAP_EXTENT_* flags for this extent */
> +	/*
> +	 * logical offset in bytes for the start of
> +	 * the extent from the beginning of the file
> +	 */
> +	__u64 fe_logical;
> +	/*
> +	 * physical offset in bytes for the start
> +	 * of the extent from the beginning of the disk
> +	 */
> +	__u64 fe_physical;
> +	/* logical length in bytes for this extent */
> +	__u64 fe_logical_length;

Or why not just leave the field name the same since the "logical length
in bytes" comment is present both here in the header and again in the
documentation?

--D

> +	/* physical length in bytes for this extent */
> +	__u64 fe_physical_length;
> +	__u64 fe_reserved64[1];
> +	/* FIEMAP_EXTENT_* flags for this extent */
> +	__u32 fe_flags;
>  	__u32 fe_reserved[3];
>  };
>  
> @@ -66,5 +82,7 @@ struct fiemap {
>  						    * merged for efficiency. */
>  #define FIEMAP_EXTENT_SHARED		0x00002000 /* Space shared with other
>  						    * files. */
> +#define FIEMAP_EXTENT_HAS_PHYS_LEN	0x00004000 /* Physical length is valid
> +						    * and set by FS. */
>  
>  #endif /* _UAPI_LINUX_FIEMAP_H */
> -- 
> 2.43.0
> 
>