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 <redacted>

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu