On Mon, Nov 22, 2021 at 04:01:02PM -0800, Mina Almasry wrote: > Add PM_THP_MAPPED 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_THP_MAPPED 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 because it must read the whole memory range > rather than a small range we care about. 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_THP_MAPPED flag at those > virtual addresses. > > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx> Acked-by: Peter Xu <peterx@xxxxxxxxxx> -- Peter Xu
