29 lines
1.5 KiB
Text
29 lines
1.5 KiB
Text
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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 plan is to deprecate the syscall and replace it with an emulation.
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The emulation will create new VMAs instead of nonlinear mappings. It's
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going to work slower for rare users of remap_file_pages() but ABI is
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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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