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Commit e9138142 authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
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btrfs: extent-tree: Add comment for inc_block_group_ro() 



inc_block_group_ro() is only designed to mark one block group read-only,
it doesn't really care if other block groups have enough free space to
contain the used space in the block group.

However due to the close connection between this function and
relocation, sometimes we can be confused and think this function is
responsible for balance space reservation, which is not true.

Add some comment to make the functionality clear.

Reviewed-by: default avatarJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 135da976

fs/btrfs/extent-tree.c

+19 −0
Original line number Diff line number Diff line
@@ -7341,6 +7341,19 @@ static u64 update_block_group_flags(struct btrfs_fs_info *fs_info, u64 flags) 
	return flags;

}



/*

 * Mark block group @cache read-only, so later write won't happen to block

 * group @cache.

 *

 * If @force is not set, this function will only mark the block group readonly

 * if we have enough free space (1M) in other metadata/system block groups.

 * If @force is not set, this function will mark the block group readonly

 * without checking free space.

 *

 * NOTE: This function doesn't care if other block groups can contain all the

 * data in this block group. That check should be done by relocation routine,

 * not this function.

 */

static int inc_block_group_ro(struct btrfs_block_group_cache *cache, int force)

{

	struct btrfs_space_info *sinfo = cache->space_info;
@@ -7374,6 +7387,12 @@ static int inc_block_group_ro(struct btrfs_block_group_cache *cache, int force) 
		    cache->bytes_super - btrfs_block_group_used(&cache->item);

	sinfo_used = btrfs_space_info_used(sinfo, true);



	/*

	 * sinfo_used + num_bytes should always <= sinfo->total_bytes.

	 *

	 * Here we make sure if we mark this bg RO, we still have enough

	 * free space as buffer (if min_allocable_bytes is not 0).

	 */

	if (sinfo_used + num_bytes + min_allocable_bytes <=

	    sinfo->total_bytes) {

		sinfo->bytes_readonly += num_bytes;