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btrfs: update documentation of set/get helpers 

Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2020-05-06 20:54:13 +02:00
parent f472d3c283
commit 583e4a2384
1 changed files with 17 additions and 13 deletions
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30
fs/btrfs/struct-funcs.c
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@ -39,23 +39,27 @@ static bool check_setget_bounds(const struct extent_buffer *eb,
}
/*
* this is some deeply nasty code.
* Macro templates that define helpers to read/write extent buffer data of a
* given size, that are also used via ctree.h for access to item members by
* specialized helpers.
*
* The end result is that anyone who #includes ctree.h gets a
* declaration for the btrfs_set_foo functions and btrfs_foo functions,
* which are wrappers of btrfs_set_token_#bits functions and
* btrfs_get_token_#bits functions, which are defined in this file.
* Generic helpers:
* - btrfs_set_8 (for 8/16/32/64)
* - btrfs_get_8 (for 8/16/32/64)
*
* These setget functions do all the extent_buffer related mapping
* required to efficiently read and write specific fields in the extent
* buffers. Every pointer to metadata items in btrfs is really just
* an unsigned long offset into the extent buffer which has been
* cast to a specific type. This gives us all the gcc type checking.
* Generic helpers with a token (cached address of the most recently accessed
* page):
* - btrfs_set_token_8 (for 8/16/32/64)
* - btrfs_get_token_8 (for 8/16/32/64)
*
* The extent buffer api is used to do the page spanning work required to
* have a metadata blocksize different from the page size.
* The set/get functions handle data spanning two pages transparently, in case
* metadata block size is larger than page. Every pointer to metadata items is
* an offset into the extent buffer page array, cast to a specific type. This
* gives us all the type checking.
*
* There are 2 variants defined, one with a token pointer and one without.
* The extent buffer pages stored in the array pages do not form a contiguous
* phyusical range, but the API functions assume the linear offset to the range
* from 0 to metadata node size.
*/
#define DEFINE_BTRFS_SETGET_BITS(bits) \