On 4/20/25 6:33 PM, John Groves wrote:
> From: John Groves <John@xxxxxxxxxx>
>
> This describes the fmap metadata - both simple and interleaved
>
> Signed-off-by: John Groves <john@xxxxxxxxxx>
> ---
> fs/fuse/famfs_kfmap.h | 90 ++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 85 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
> index 325adb8b99c5..7c8d57b52e64 100644
> --- a/fs/fuse/famfs_kfmap.h
> +++ b/fs/fuse/famfs_kfmap.h
> @@ -7,10 +7,90 @@
> #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 (devined in
divined
> + * 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 retrievable via the GET_DAXDEV
must be
> + * 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
Combine 2 lines above.
> + * * 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).
> + */
> +
> +
> /*
> - * These structures are the in-memory metadata format for famfs files. Metadata
> - * retrieved via the GET_FMAP response is converted to this format for use in
> - * resolving file mapping faults.
> + * The structures below are the in-memory metadata format for famfs files.
> + * Metadata retrieved via the GET_FMAP response is converted to this format
> + * for use in * resolving file mapping faults.
^drop
> + *
> + * The GET_FMAP response contains the same information, but in a more
> + * message-and-versioning-friendly format. Those structs can be found in the
> + * famfs section of include/uapi/linux/fuse.h (aka fuse_kernel.h in libfuse)
> */
>
> enum famfs_file_type {
> @@ -19,7 +99,7 @@ enum famfs_file_type {
> FAMFS_LOG,
> };
>
> -/* We anticipate the possiblity of supporting additional types of extents */
> +/* We anticipate the possibility of supporting additional types of extents */
> enum famfs_extent_type {
> SIMPLE_DAX_EXTENT,
> INTERLEAVED_EXTENT,
> @@ -63,7 +143,7 @@ struct famfs_file_meta {
> /*
> * dax_devlist
> *
> - * This is the in-memory daxdev metadata that is populated by
> + * This is the in-memory daxdev metadata that is populated by parsing
> * the responses to GET_FMAP messages
> */
> struct famfs_daxdev {
--
~Randy
