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| -rw-r--r-- | Documentation/vm/remap_file_pages.rst | 33 |
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diff --git a/Documentation/vm/remap_file_pages.rst b/Documentation/vm/remap_file_pages.rst new file mode 100644 index 000000000..7bef6718e --- /dev/null +++ b/Documentation/vm/remap_file_pages.rst @@ -0,0 +1,33 @@ +.. _remap_file_pages: + +============================== +remap_file_pages() system call +============================== + +The remap_file_pages() system call is used to create a nonlinear mapping, +that is, a mapping in which the pages of the file are mapped into a +nonsequential order in memory. The advantage of using remap_file_pages() +over using repeated calls to mmap(2) is that the former approach does not +require the kernel to create additional VMA (Virtual Memory Area) data +structures. + +Supporting of nonlinear mapping requires significant amount of non-trivial +code in kernel virtual memory subsystem including hot paths. Also to get +nonlinear mapping work kernel need a way to distinguish normal page table +entries from entries with file offset (pte_file). Kernel reserves flag in +PTE for this purpose. PTE flags are scarce resource especially on some CPU +architectures. It would be nice to free up the flag for other usage. + +Fortunately, there are not many users of remap_file_pages() in the wild. +It's only known that one enterprise RDBMS implementation uses the syscall +on 32-bit systems to map files bigger than can linearly fit into 32-bit +virtual address space. This use-case is not critical anymore since 64-bit +systems are widely available. + +The syscall is deprecated and replaced it with an emulation now. The +emulation creates new VMAs instead of nonlinear mappings. It's going to +work slower for rare users of remap_file_pages() but ABI is preserved. + +One side effect of emulation (apart from performance) is that user can hit +vm.max_map_count limit more easily due to additional VMAs. See comment for +DEFAULT_MAX_MAP_COUNT for more details on the limit. |