Re: [PATCH v2 0/2] sys/prctl: expose TASK_SIZE value to userspace

From: Waiman Long
Date: Thu May 02 2019 - 17:02:12 EST

On 5/2/19 4:52 PM, 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 others
> to do the same.
> 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 | 10 ++++++++++
> 2 files changed, 13 insertions(+)
> man2/prctl.2 | 9 +++++++++
> 1 file changed, 9 insertions(+)
> --
> 2.18.1

What did you change in v2 versus v1?