On Wed, 10 Nov 2021 14:11:20 -0800 Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > Add PM_HUGE_THP MAPPING to allow userspace to detect whether a given virt > address is currently mapped by a transparent huge page or not. Example > use case is a process requesting THPs from the kernel (via a huge tmpfs > mount for example), for a performance critical region of memory. The > userspace may want to query whether the kernel is actually backing this > memory by hugepages or not. > > PM_HUGE_THP_MAPPING bit is set if the virt address is mapped at the PMD > level and the underlying page is a transparent huge page. > > A few options were considered: > 1. Add /proc/pid/pageflags that exports the same info as > /proc/kpageflags. This is not appropriate because many kpageflags are > inappropriate to expose to userspace processes. > 2. Simply get this info from the existing /proc/pid/smaps interface. > There are a couple of issues with that: > 1. /proc/pid/smaps output is human readable and unfriendly to > programatically parse. > 2. /proc/pid/smaps is slow. The cost of reading /proc/pid/smaps into > userspace buffers is about ~800us per call, and this doesn't > include parsing the output to get the information you need. The > cost of querying 1 virt address in /proc/pid/pagemaps however is > around 5-7us. > > Tested manually by adding logging into transhuge-stress, and by > allocating THP and querying the PM_HUGE_THP_MAPPING flag at those > virtual addresses. > > --- a/tools/testing/selftests/vm/transhuge-stress.c > +++ b/tools/testing/selftests/vm/transhuge-stress.c > @@ -16,6 +16,12 @@ > #include <string.h> > #include <sys/mman.h> > > +/* > + * We can use /proc/pid/pagemap to detect whether the kernel was able to find > + * hugepages or no. This can be very noisy, so is disabled by default. > + */ > +#define NO_DETECT_HUGEPAGES > + > > ... > > +#ifndef NO_DETECT_HUGEPAGES > + if (!PAGEMAP_THP(ent[0])) > + fprintf(stderr, "WARNING: detected non THP page\n"); > +#endif This looks like a developer thing. Is there any point in leaving it in the mainline code?
