Re: [PATCH V7 18/19] famfs_fuse: Add famfs fmap metadata documentation

Dave Jiang
2026-02-19T20:22:41Z

On 1/18/26 3:34 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
>
> This describes the fmap metadata - both simple and interleaved
>
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>

> ---
> fs/fuse/famfs_kfmap.h | 73 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 73 insertions(+)
>
> diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
> index 0fff841f5a9e..970ad802b492 100644
> --- a/fs/fuse/famfs_kfmap.h
> +++ b/fs/fuse/famfs_kfmap.h
> @@ -7,6 +7,79 @@
> #ifndef FAMFS_KFMAP_H
> #define FAMFS_KFMAP_H
>
> +/* KABI version 43 (aka v2) fmap structures
> + *
> + * The location of the memory backing for a famfs file is described by
> + * the response to the GET_FMAP fuse message (defined in
> + * include/uapi/linux/fuse.h
> + *
> + * There are currently two extent formats: Simple and Interleaved.
> + *
> + * Simple extents are just (devindex, offset, length) tuples, where devindex
> + * references a devdax device that must be retrievable via the GET_DAXDEV
> + * message/response.
> + *
> + * The extent list size must be >= file_size.
> + *
> + * Interleaved extents merit some additional explanation. Interleaved
> + * extents stripe data across a collection of strips. Each strip is a
> + * contiguous allocation from a single devdax device - and is described by
> + * a simple_extent structure.
> + *
> + * Interleaved_extent example:
> + * ie_nstrips = 4
> + * ie_chunk_size = 2MiB
> + * ie_nbytes = 24MiB
> + *
> + * ┌────────────┐────────────┐────────────┐────────────┐
> + * │Chunk = 0 │Chunk = 1 │Chunk = 2 │Chunk = 3 │
> + * │Strip = 0 │Strip = 1 │Strip = 2 │Strip = 3 │
> + * │Stripe = 0 │Stripe = 0 │Stripe = 0 │Stripe = 0 │
> + * │ │ │ │ │
> + * └────────────┘────────────┘────────────┘────────────┘
> + * │Chunk = 4 │Chunk = 5 │Chunk = 6 │Chunk = 7 │
> + * │Strip = 0 │Strip = 1 │Strip = 2 │Strip = 3 │
> + * │Stripe = 1 │Stripe = 1 │Stripe = 1 │Stripe = 1 │
> + * │ │ │ │ │
> + * └────────────┘────────────┘────────────┘────────────┘
> + * │Chunk = 8 │Chunk = 9 │Chunk = 10 │Chunk = 11 │
> + * │Strip = 0 │Strip = 1 │Strip = 2 │Strip = 3 │
> + * │Stripe = 2 │Stripe = 2 │Stripe = 2 │Stripe = 2 │
> + * │ │ │ │ │
> + * └────────────┘────────────┘────────────┘────────────┘
> + *
> + * * Data is laid out across chunks in chunk # order
> + * * Columns are strips
> + * * Strips are contiguous devdax extents, normally each coming from a
> + * different memory device
> + * * Rows are stripes
> + * * The number of chunks is (int)((file_size + chunk_size - 1) / chunk_size)
> + * (and obviously the last chunk could be partial)
> + * * The stripe_size = (nstrips * chunk_size)
> + * * chunk_num(offset) = offset / chunk_size //integer division
> + * * strip_num(offset) = chunk_num(offset) % nchunks
> + * * stripe_num(offset) = offset / stripe_size //integer division
> + * * ...You get the idea - see the code for more details...
> + *
> + * Some concrete examples from the layout above:
> + * * Offset 0 in the file is offset 0 in chunk 0, which is offset 0 in
> + * strip 0
> + * * Offset 4MiB in the file is offset 0 in chunk 2, which is offset 0 in
> + * strip 2
> + * * Offset 15MiB in the file is offset 1MiB in chunk 7, which is offset
> + * 3MiB in strip 3
> + *
> + * Notes about this metadata format:
> + *
> + * * For various reasons, chunk_size must be a multiple of the applicable
> + * PAGE_SIZE
> + * * Since chunk_size and nstrips are constant within an interleaved_extent,
> + * resolving a file offset to a strip offset within a single
> + * interleaved_ext is order 1.
> + * * If nstrips==1, a list of interleaved_ext structures degenerates to a
> + * regular extent list (albeit with some wasted struct space).
> + */
> +
> /*
> * The structures below are the in-memory metadata format for famfs files.
> * Metadata retrieved via the GET_FMAP response is converted to this format