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