Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap

From: Peter Xu
Date: Mon Nov 22 2021 - 20:10:55 EST

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