mirror of https://gitee.com/openkylin/linux.git
29 lines
1.5 KiB
Plaintext
29 lines
1.5 KiB
Plaintext
|
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 plan is to deprecate the syscall and replace it with an emulation.
|
||
|
|
The emulation will create 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.
|