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. 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
