On Fri, May 03, 2019 at 01:49:12PM -0700, Yury Norov wrote: > 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? >
there's no real cost in exposing the value that is known to the kernel, anyways, as long as it's not a freaking hassle (like trying to go with this prctl(2) stunt). We just need to get it properly exported alongside other task's VM-related values at /proc/<pid>/status. > 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
