Hi Kirill,
On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov
<[email protected]> 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 <[email protected]>
> ---
> 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 [email protected]. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
--
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
Michael Kerrisk wrote:
> Hi Kirill,
>
> On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov
> <[email protected]> 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.
--
Kirill A. Shutemov
Hi Kirill,
On Thu, Jun 12, 2014 at 11:40 AM, Kirill A. Shutemov
<[email protected]> wrote:
> Michael Kerrisk wrote:
>> Hi Kirill,
>>
>> On Thu, May 8, 2014 at 2:41 PM, Kirill A. Shutemov
>> <[email protected]> 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/