Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.
1 The remap_file_pages() system call is used to create a nonlinear mapping, 2 that is, a mapping in which the pages of the file are mapped into a 3 nonsequential order in memory. The advantage of using remap_file_pages() 4 over using repeated calls to mmap(2) is that the former approach does not 5 require the kernel to create additional VMA (Virtual Memory Area) data 6 structures. 7 8 Supporting of nonlinear mapping requires significant amount of non-trivial 9 code in kernel virtual memory subsystem including hot paths. Also to get 10 nonlinear mapping work kernel need a way to distinguish normal page table 11 entries from entries with file offset (pte_file). Kernel reserves flag in 12 PTE for this purpose. PTE flags are scarce resource especially on some CPU 13 architectures. It would be nice to free up the flag for other usage. 14 15 Fortunately, there are not many users of remap_file_pages() in the wild. 16 It's only known that one enterprise RDBMS implementation uses the syscall 17 on 32-bit systems to map files bigger than can linearly fit into 32-bit 18 virtual address space. This use-case is not critical anymore since 64-bit 19 systems are widely available. 20 21 The syscall is deprecated and replaced it with an emulation now. The 22 emulation creates new VMAs instead of nonlinear mappings. It's going to 23 work slower for rare users of remap_file_pages() but ABI is preserved. 24 25 One side effect of emulation (apart from performance) is that user can hit 26 vm.max_map_count limit more easily due to additional VMAs. See comment for 27 DEFAULT_MAX_MAP_COUNT for more details on the limit.