erofs: support parsing big pcluster compact indexes
Different from non-compact indexes, several lclusters are packed as the compact form at once and an unique base blkaddr is stored for each pack, so each lcluster index would take less space on avarage (e.g. 2 bytes for COMPACT_2B.) btw, that is also why BIG_PCLUSTER switch should be consistent for compact head0/1. Prior to big pcluster, the size of all pclusters was 1 lcluster. Therefore, when a new HEAD lcluster was scanned, blkaddr would be bumped by 1 lcluster. However, that way doesn't work anymore for big pcluster since we actually don't know the compressed size of pclusters in advance (before reading CBLKCNT lcluster). So, instead, let blkaddr of each pack be the first pcluster blkaddr with a valid CBLKCNT, in detail, 1) if CBLKCNT starts at the pack, this first valid pcluster is itself, e.g. _____________________________________________________________ |_CBLKCNT0_|_NONHEAD_| .. |_HEAD_|_CBLKCNT1_| ... |_HEAD_| ... ^ = blkaddr base ^ += CBLKCNT0 ^ += CBLKCNT1 2) if CBLKCNT doesn't start at the pack, the first valid pcluster is the next pcluster, e.g. _________________________________________________________ | NONHEAD_| .. |_HEAD_|_CBLKCNT0_| ... |_HEAD_|_HEAD_| ... ^ = blkaddr base ^ += CBLKCNT0 ^ += 1 When a CBLKCNT is found, blkaddr will be increased by CBLKCNT lclusters, or a new HEAD is found immediately, bump blkaddr by 1 instead (see the picture above.) Also noted if CBLKCNT is the end of the pack, instead of storing delta1 (distance of the next HEAD lcluster) as normal NONHEADs, it still uses the compressed block count (delta0) since delta1 can be calculated indirectly but the block count can't. Adjust decoding logic to fit big pcluster compact indexes as well. Link: https://lore.kernel.org/r/20210407043927.10623-9-xiang@kernel.org Acked-by:Chao Yu <yuchao0@huawei.com> Signed-off-by:
Gao Xiang <hsiangkao@redhat.com>
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