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

[Rafael Aquini]

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