Hi Kirill, On Thu, Jun 12, 2014 at 11:40 AM, Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > 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 con‐ >> sider migrating to alternatives. >> >> Okay? > > Yep. Looks okay to me. Thanks for checking. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
