Michael Kerrisk wrote: > Hi Kirill, > > On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > 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. > > Best to CC linux-api@ > (https://www.kernel.org/doc/man-pages/linux-api-ml.html) on patches > like this, as well as the man-pages maintainer, so that something goes > into the man page. I added the following into the man page: > > Note: this system call is (since Linux 3.16) deprecated and > will eventually be replaced by a slower in-kernel emulation. > Those few applications that use this system call should conâ?? > sider migrating to alternatives. > > Okay? Yep. Looks okay to me. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>