Re: [PATCH 04/19] btrfs: keep track of cleanliness of the bitmap

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Oct 07, 2019 at 04:17:35PM -0400, Dennis Zhou wrote:
> There is a cap in btrfs in the amount of free extents that a block group
> can have. When it surpasses that threshold, future extents are placed
> into bitmaps. Instead of keeping track of if a certain bit is trimmed or
> not in a second bitmap, keep track of the relative state of the bitmap.
> 
> With async discard, trimming bitmaps becomes a more frequent operation.
> As a trade off with simplicity, we keep track of if discarding a bitmap
> is in progress. If we fully scan a bitmap and trim as necessary, the
> bitmap is marked clean. This has some caveats as the min block size may
> skip over regions deemed too small. But this should be a reasonable
> trade off rather than keeping a second bitmap and making allocation
> paths more complex. The downside is we may overtrim, but ideally the min
> block size should prevent us from doing that too often and getting stuck
> trimming
> pathological cases.
> 
> BTRFS_FSC_TRIMMING_BITMAP is added to indicate a bitmap is in the
> process of being trimmed. If additional free space is added to that
> bitmap, the bit is cleared. A bitmap will be marked BTRFS_FSC_TRIMMED if
> the trimming code was able to reach the end of it and the former is
> still set.
> 
> Signed-off-by: Dennis Zhou <dennis@kernel.org>

I went through and looked at the end result and it appears to me that we never
have TRIMMED and TRIMMING set at the same time.  Since these are the only two
flags, and TRIMMING is only set on bitmaps, it makes more sense for this to be
more like

enum btrfs_trim_state {
	BTRFS_TRIM_STATE_TRIMMED,
	BTRFS_TRIM_STATE_TRIMMING,
	BTRFS_TRIM_STATE_UNTRIMMED,
};

and then just have enum btrfs_trim_state trim_state in the free space entry.
This makes things a bit cleaner since it's really just a state indicator rather
than a actual flags.  Thanks,

Josef