Background: On older kernel?s, we could have our device driver create char devices and implement file_operations and vm_operations for open, release, mmap, fault etc. The driver allocates memory (128KB, order 5) as compound pages. The user application would then map/mmap these device files to perform read/write operations. On recent kernels, this creates problems when the user space maps multiple 128KB chunks that exceed 2MB. This would sometime result in bad page getting mapped to the user space process. Almost always we see ?Bad page map? errors during munmap because the map count is going below 0. It looks like the culprit is zap_pte_range(), which calls page_remove_rmap() with the compound_flag = false. As a result, instead of decrementing the compound_mapcount for the page the page->_mapcount is decremented causing a lot of bad page errors. Questions: Is this the right usage of compound pages? I.e can I allocate compound pages in my kernel driver to be mapped to char device file by a user process? If yes, then why does it fail on latest kernel when the mmap-ed size exceeds 2MB? If no, why was it working on the older kernels? If it worked then, shouldn?t it work now? What is special about map size being greater than 2MB to trigger this? Should compound pages be used for Anonymous purposes only?
