1The remap_file_pages() system call is used to create a nonlinear mapping, 2that is, a mapping in which the pages of the file are mapped into a 3nonsequential order in memory. The advantage of using remap_file_pages() 4over using repeated calls to mmap(2) is that the former approach does not 5require the kernel to create additional VMA (Virtual Memory Area) data 6structures. 7 8Supporting of nonlinear mapping requires significant amount of non-trivial 9code in kernel virtual memory subsystem including hot paths. Also to get 10nonlinear mapping work kernel need a way to distinguish normal page table 11entries from entries with file offset (pte_file). Kernel reserves flag in 12PTE for this purpose. PTE flags are scarce resource especially on some CPU 13architectures. It would be nice to free up the flag for other usage. 14 15Fortunately, there are not many users of remap_file_pages() in the wild. 16It's only known that one enterprise RDBMS implementation uses the syscall 17on 32-bit systems to map files bigger than can linearly fit into 32-bit 18virtual address space. This use-case is not critical anymore since 64-bit 19systems are widely available. 20 21The syscall is deprecated and replaced it with an emulation now. The 22emulation creates new VMAs instead of nonlinear mappings. It's going to 23work slower for rare users of remap_file_pages() but ABI is preserved. 24 25One side effect of emulation (apart from performance) is that user can hit 26vm.max_map_count limit more easily due to additional VMAs. See comment for 27DEFAULT_MAX_MAP_COUNT for more details on the limit. 28