Message-ID: <f0b218f4-7379-4fa5-93e4-a1c9dd4c411c@infradead.org>
Date: Sun, 20 Apr 2025 20:51:48 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: John Groves <John@...ves.net>, Dan Williams <dan.j.williams@...el.com>,
 Miklos Szeredi <miklos@...redb.hu>, Bernd Schubert <bschubert@....com>
Cc: John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>,
 Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>,
 Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, "Darrick J . Wong"
 <djwong@...nel.org>, Luis Henriques <luis@...lia.com>,
 Jeff Layton <jlayton@...nel.org>, Kent Overstreet
 <kent.overstreet@...ux.dev>, Petr Vorel <pvorel@...e.cz>,
 Brian Foster <bfoster@...hat.com>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Amir Goldstein <amir73il@...il.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Stefan Hajnoczi <shajnocz@...hat.com>, Joanne Koong
 <joannelkoong@...il.com>, Josef Bacik <josef@...icpanda.com>,
 Aravind Ramesh <arramesh@...ron.com>, Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC PATCH 17/19] famfs_fuse: Add famfs metadata documentation



On 4/20/25 6:33 PM, John Groves wrote:
> From: John Groves <John@...ves.net>
> 
> This describes the fmap metadata - both simple and interleaved
> 
> Signed-off-by: John Groves <john@...ves.net>
> ---
>  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

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