Hi,
(Sorry, sadly I didn't seek much time working on modifying lz4, so just try to record a feature request first...)

Our next step of EROFS on lz4 is to support compression with an external dictionary.
Test results show that it can have benefits on some files with small cluster sizes (which have small
I/O amplication) and specific patterns.

The compressor side looks good. However, it seems that a partial decompression API like
LZ4_decompress_safe_partial_usingDict() is missing...

It would be much helpful to get this so that we don't need to take more unnecessary page cache...

Thanks,
Gao Xiang