remap_file_pages.rst (1671B)
1.. _remap_file_pages: 2 3============================== 4remap_file_pages() system call 5============================== 6 7The remap_file_pages() system call is used to create a nonlinear mapping, 8that is, a mapping in which the pages of the file are mapped into a 9nonsequential order in memory. The advantage of using remap_file_pages() 10over using repeated calls to mmap(2) is that the former approach does not 11require the kernel to create additional VMA (Virtual Memory Area) data 12structures. 13 14Supporting of nonlinear mapping requires significant amount of non-trivial 15code in kernel virtual memory subsystem including hot paths. Also to get 16nonlinear mapping work kernel need a way to distinguish normal page table 17entries from entries with file offset (pte_file). Kernel reserves flag in 18PTE for this purpose. PTE flags are scarce resource especially on some CPU 19architectures. It would be nice to free up the flag for other usage. 20 21Fortunately, there are not many users of remap_file_pages() in the wild. 22It's only known that one enterprise RDBMS implementation uses the syscall 23on 32-bit systems to map files bigger than can linearly fit into 32-bit 24virtual address space. This use-case is not critical anymore since 64-bit 25systems are widely available. 26 27The syscall is deprecated and replaced it with an emulation now. The 28emulation creates new VMAs instead of nonlinear mappings. It's going to 29work slower for rare users of remap_file_pages() but ABI is preserved. 30 31One side effect of emulation (apart from performance) is that user can hit 32vm.max_map_count limit more easily due to additional VMAs. See comment for 33DEFAULT_MAX_MAP_COUNT for more details on the limit.