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? Cheers, Michael > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > --- > Documentation/vm/remap_file_pages.txt | 28 ++++++++++++++++++++++++++++ > mm/fremap.c | 4 ++++ > 2 files changed, 32 insertions(+) > create mode 100644 Documentation/vm/remap_file_pages.txt > > diff --git a/Documentation/vm/remap_file_pages.txt b/Documentation/vm/remap_file_pages.txt > new file mode 100644 > index 000000000000..560e4363a55d > --- /dev/null > +++ b/Documentation/vm/remap_file_pages.txt > @@ -0,0 +1,28 @@ > +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. > diff --git a/mm/fremap.c b/mm/fremap.c > index 34feba60a17e..12c3bb63b7f9 100644 > --- a/mm/fremap.c > +++ b/mm/fremap.c > @@ -152,6 +152,10 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, > int has_write_lock = 0; > vm_flags_t vm_flags = 0; > > + pr_warn_once("%s (%d) uses depricated remap_file_pages() syscall. " > + "See Documentation/vm/remap_file_pages.txt.\n", > + current->comm, current->pid); > + > if (prot) > return err; > /* > -- > 2.0.0.rc2 > > -- > 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> -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface", http://blog.man7.org/ -- 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
