IBM Spectrum Scale(GPFS) Alert for V4.2 and V5.0 levels: data corruption may happen on compressed files.

Generally, the compression operation will be skipped or fail if a regular file is already memory-mapped. Conversely, if the file has already been compressed before a memory-map operation happens on the same file, the data area being mmap written will first be decompressed. However, when a regular file is being compressed and is then memory-mapped, the mmap read operation could page the data into the Operating System (OS) page-cache before a data block is compressed. If an mmap write operation on the just paged-in page happens at the same time as that data block is being compressed, the newly written data in the page-cache may be written to that compressed data block directly without a decompression being attempted. Such a compressed data block containing some uncompressed data would result in decompression failure with an EIO error being returned in most cases to users or applications. Although possible but less likely, corrupted data undetected by users may be returned, as damaged compression data could be mistakenly interpreted by the decompression process. In addition, an FSErrBadCompressBlock file system structure error may be logged into the system log file.