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![Mike Rapoport (IBM)](/assets/img/avatar_default.png)
It is enough to use a file name to cross-reference another rst document. Jon says: The right things will happen in the HTML output, readers of the plain-text will know immediately where to go, and we don't have to add the label clutter. Drop reference markup and unnecessary labels and use plain file names. Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org> Link: https://lore.kernel.org/r/20230201094156.991542-3-rppt@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
31 lines
1.6 KiB
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31 lines
1.6 KiB
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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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