[PATCH V4 18/19] famfs_fuse: Add famfs fmap metadata documentation

[John Groves <John@Groves.net>]

From: John Groves <John@Groves.net>

This describes the fmap metadata - both simple and interleaved

Signed-off-by: John Groves <john@groves.net>
---
 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
-- 
2.52.0