Re: [PATCH v2 3/4] btrfs: introduce btrfs_subpage_bitmap_info

[Nikolay Borisov <nborisov@suse.com>]

On 17.08.21 г. 12:38, Qu Wenruo wrote:
> Currently we use fixed size u16 bitmap for subpage bitmap.
> This is fine for 4K sectorsize with 64K page size.
> 
> But for 4K sectorsize and larger page size, the bitmap is too small,
> while for smaller page size like 16K, u16 bitmaps waste too much space.
> 
> Here we introduce a new helper structure, btrfs_subpage_bitmap_info, to
> record the proper bitmap size, and where each bitmap should start at.
> 
> By this, we can later compact all subpage bitmaps into one u32 bitmap.
> 
> This patch is the first step towards such compact bitmap.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/ctree.h   |  1 +
>  fs/btrfs/disk-io.c | 12 +++++++++---
>  fs/btrfs/subpage.c | 35 +++++++++++++++++++++++++++++++++++
>  fs/btrfs/subpage.h | 36 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 81 insertions(+), 3 deletions(-)
> 

<snip>

> +/*
> + * Extra info for subpapge bitmap.
> + *
> + * For subpage we pack all uptodate/error/dirty/writeback/ordered
> + * bitmaps into one larger bitmap.
> + *
> + * This structure records how they are organized in such bitmap:
> + *
> + * /- uptodate_offset	/- error_offset	/- dirty_offset
> + * |			|		|
> + * v			v		v
> + * |u|u|u|u|........|u|u|e|e|.......|e|e| ...	|o|o|

nit: the 'e' that the dirty offset is pointing to should be a 'd', I'm
sure David can fix this while merging.

<snip>