Re: [PATCH 1/2] mm: mark remap_file_pages() syscall as deprecated

[Michael Kerrisk <mtk.manpages@gmail.com>]

Hi Kirill,

On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> 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@linux.intel.com>
> ---
>  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
>
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/