Re: [PATCH v3 0/2] sys/prctl: expose TASK_SIZE value to userspace

[Yury Norov]

On Fri, May 03, 2019 at 02:10:19PM -0400, Joel Savitz wrote:
> In the mainline kernel, there is no quick mechanism to get the virtual
> memory size of the current process from userspace.
> 
> Despite the current state of affairs, this information is available to the
> user through several means, one being a linear search of the entire address
> space. This is an inefficient use of cpu cycles.
> 
> A component of the libhugetlb kernel test does exactly this, and as
> systems' address spaces increase beyond 32-bits, this method becomes
> exceedingly tedious.
> 
> For example, on a ppc64le system with a 47-bit address space, the linear
> search causes the test to hang for some unknown amount of time. I
> couldn't give you an exact number because I just ran it for about 10-20
> minutes and went to go do something else, probably to get coffee or
> something, and when I came back, I just killed the test and patched it
> to use this new mechanism. I re-ran my new version of the test using a
> kernel with this patch, and of course it passed through the previously
> bottlenecking codepath nearly instantaneously.
> 
> As such, I propose that the prctl syscall be extended to include the
> option to retrieve TASK_SIZE from the kernel.
> 
> This patch will allow us to upgrade an O(n) codepath to O(1) in an
> architecture-independent manner, and provide a mechanism for future
> generations to do the same.

So the only reason for the new API is boosting some random poorly
written userspace test? Why don't you introduce binary search instead?

Look at /proc/<pid>/maps. It may help to reduce the memory area to be
checked.
 
> Changes from v2:
>  We now account for the case of 32-bit compat userspace on a 64-bit kernel
>  More detail about the nature of TASK_SIZE in documentation
> 
> Joel Savitz(2):
>   sys/prctl: add PR_GET_TASK_SIZE option to prctl(2)
>   prctl.2: Document the new PR_GET_TASK_SIZE option
> 
>  include/uapi/linux/prctl.h |  3 +++
>  kernel/sys.c               | 23 +++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
>  man2/prctl.2 | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> --
> 2.18.1