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