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.
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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.
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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.
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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.
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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.
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